A&A 410, 481-509 (2003)
DOI: 10.1051/0004-6361:20031147
K. G. Noeske - P. Papaderos - L. M. Cairós - K. J. Fricke
Universitäts-Sternwarte, Geismarlandstraße 11, 37083 Göttingen, Germany
Received 8 October 2002 / Accepted 22 July 2003
Abstract
We have analyzed deep Near Infrared (NIR) broad band images
for a sample of Blue Compact Dwarf Galaxies (BCDs), observed with the
ESO NTT
and Calar Alto
3.6 m
telescopes.
The data presented here allows for the detection and quantitative
study of the extended stellar low-surface brightness (LSB) host
galaxy in all sample BCDs.
NIR surface brightness profiles (SBPs) of the LSB host galaxies
agree at large galactocentric radii with those from optical studies,
showing also an exponential intensity decrease and compatible scale lengths.
At small to intermediate radii (within 1-3 exponential scale lengths), however,
the NIR data reveals for more than one half of our sample BCDs evidence for
a significant flattening of the exponential profile of the LSB component.
Such profiles (type V SBPs, Binggeli & Cameron #!binggeli91!#) have rarely been
detected in the LSB component of BCDs at optical wavelengths, where
the relative flux contribution of the starburst, being stronger than in
the NIR, can readily hide a possible central intensity depression in
the underlying LSB host.
The structural properties, frequency and physical origin of type V LSB profiles in
BCDs and dwarf galaxies in general have not yet been subject
to systematic studies.
Nevertheless, the occurrence of such profiles
in an appreciable fraction of BCDs would impose important
new observational constraints to the radial mass distribution
of the stellar LSB component, as well as to the photometric
fading of these systems after the termination of star-forming
activities.
We test the suitability of two empirical fitting functions,
a modified exponential distribution (Papaderos et al. #!papaderos96a!#)
and the Sérsic law, for the systematization of the structural
properties of BCD host galaxies which show a type V intensity
distribution. Either function has been found to satisfactorily
fit a type V distribution.
However, it is argued that the practical applicability
of Sérsic fits to the LSB emission of BCDs is limited by the
extreme sensitivity of the achieved solutions to,
e.g., small uncertainties in the sky subtraction
and SBP derivation.
We find that most of the sample BCDs show in their stellar LSB host galaxy optical-NIR
colors indicative of an evolved stellar population with subsolar metallicity.
Unsharp-masked NIR maps reveal numerous morphological details and
indicate in some cases, in combination with optical data,
appreciable non-uniform dust absorption
on a spatial scale as large as
1 kpc.
Key words: galaxies: dwarf - galaxies: evolution - galaxies: structure - galaxies: starburst
The star-formation history and chemodynamic evolution of
Blue Compact Dwarf (BCD) galaxies are central issues in
the contemporary dwarf galaxy research. In spite of being old
in their vast majority, BCDs resemble in many aspects unevolved
low-mass galaxies in the early Universe.
They are gas-rich (H I mass fraction of typically >30%) and
metal-deficient (/50
/3) extragalactic systems,
undergoing intense, spatially extended star-forming (SF) activity.
Such properties are believed to have been common among young low-mass
objects at high to intermediate redshift, such as pre-galactic
building blocks (Lowenthal et al. 1997; Hirashita et al. 2000; Fujita et al. 2001) or the
progenitors of the present-day dwarf spheroidals (e.g. Babul & Rees
1992; Guzmán et al. 1998). BCDs are therefore
convenient nearby laboratories to study at high spatial resolution the
impact of collective star formation on the spectrophotometric and chemodynamic
properties of these distant and faint extragalactic sources. Moreover,
they are important testbeds for deducing constraints to cosmological parameters,
such as the primordial 4He abundance ratio, and to monitor the
synthesis and dispersal of heavy elements in a nearly pristine
environment (Peimbert & Torres-Peimbert 1974; Pagel et al. 1992; Izotov et al. 1997).
The understanding of the origin and implications of the starburst phenomenon in BCDs is necessary for elucidating evolutionary pathways of dwarf galaxies (DGs) in general. Are BCDs active phases in the lifetime of dormant dwarf irregulars (dIs) and do the latter fade to dwarf ellipticals (dEs) once their gas reservoir has been depleted (see e.g. Lin & Faber 1983; Thuan 1985; Silk et al. 1987; Davies & Phillipps 1988)? What is the role of the environment (e.g., Babul & Rees 1992; Pustil'nik et al. 2001) and of Dark Matter (DM; Dekel & Silk 1986; Ferrara & Tolstoy 2000), and does the latter invariably dominate the mass within the Holmberg radius of a BCD (Papaderos et al. 1996b; hereafter P96b)? Do most BCDs undergo intermittent bursts or rather prolonged periods of elevated star formation (Vallenari & Bomans 1996; Noguchi 2001; Rieschick & Hensler 2001; Schulte-Ladbeck et al. 2001)? Despite much previous effort, the observational evidence available thus far is still too fragmentary to allow for unambiguous answers to the aforementioned questions.
Recent studies suggest, however, that key information for assessing DG
evolution can be inferred from studies of the stellar low-surface
brightness (LSB) host galaxy of these systems. In BCDs, the LSB
component, underlying the SF regions, has first been disclosed through
deep CCD imaging by Loose & Thuan (1986, hereafter LT86),
and has in the following been confirmed and further studied by various
authors (cf., e.g., the list given later in this section). This
extended stellar host was found to account for 1/2 of the light
inside the 25 B mag/
isophote (P96b, Salzer &
Norton 1999), and
to typically dominate
the intensity and color distribution of BCDs for
24.5 B mag/
.
Such an
evenly distributed, evolved stellar component is observed in all
types of DGs, except for the extremely rare type of i0 BCDs in the
classification scheme of LT86. Its red colors and smooth
appearance in the main class of iE/nE BCDs (LT86) indicate
that these systems are several Gyr old, gas-rich DGs, having not been
forming stars at the presently large rate throughout their lifetime.
Different lines of evidence, outlined in the following, suggest that
elaborate studies of the structural and kinematic properties of the
LSB component are fundamental to assess at least two
central issues of DG research: the evolutionary connections between
DGs and the regulation of the SF process in these systems.
According to the standard evolutionary hypothesis for dwarf galaxies, dIs, dEs and BCDs differ basically by their gas content and the amplitude of their ongoing SF activity (Thuan 1985; Davies & Phillipps 1988). One would therefore expect that, on average, the evolved stellar LSB host in all these three main DG classes is indistinguishable from one another with respect to its structural properties. However, P96b and subsequent authors (Patterson & Thuan 1996; Marlowe et al. 1997; Salzer & Norton 1999, see also Papaderos et al. 2002) found that, at equal absolute magnitude, the stellar LSB component of iE/nE BCDs is systematically more compact than other types of DGs. P96b interpreted this structural disparity as the result of adiabatic contraction of the stellar LSB component of BCDs in response to a large-scale gas inflow, preceding the ignition of a starburst in a BCD. Quite interestingly, subsequent interferometric H I studies have shown that the gaseous halo of BCDs is by at least a factor of two more centrally concentrated than in dIs (van Zee et al. 1998, 2001; see also Simpson et al. 2000), lending circumstantial support to the latter hypothesis. Whether or not such observational constraints can be accounted for by the dynamical evolution of the stellar and gaseous matter in DGs (P96b) or, alternatively, an extraordinarily dense Dark Matter halo, being particular to BCD galaxies (Meurer et al. 1998) awaits to be investigated by numerical 3D-simulations. These are also needed to explore a possible connection between the LSB morphology and the evolutionary state of BCDs, as proposed by Noeske et al. (2000).
The evolved LSB component contains, due to its high M/L, the bulk of the stellar matter in a BCD. Thus, provided that DM does not dominate within the optical radius, it forms, together with the gaseous component, the gravitational potential within which SF activity occurs. It is therefore worth exploring whether the structural properties of the LSB component influence the SF process in a BCD. P96b found a trend for the fractional surface area of the SF component of BCDs to decrease with increasing LSB luminosity. For a constant M/L, this trend translates into a mass-morphology connection for BCDs: SF activity in more massive BCDs occurs mainly in the inner part of the LSB component, leading to a nE morphology and surface brightness profiles (SBPs) sometimes superficially resembling a de Vaucouleurs law. Conversely, SF activity in low-mass BCDs is spread over a larger portion of the LSB host, resulting in an iE morphology and SBPs possessing a conspicuous plateau feature (cf. Papaderos et al. 1996a, hereafter P96a) at intermediate intensity levels.
Our current knowledge of the nature of the underlying
LSB component relies mainly on optical surface photometry studies
of its faint periphery
(e.g. LT86, Kunth et al. 1988, P96a, Telles et al.
1997; Marlowe et al. 1997; Doublier et al. 1997, 1999; Salzer & Norton 1999;
Vennik et al. 2000; Cairós et al. 2001a,b; Makarova et al. 2002).
In optical wavelengths, extended stellar and gaseous starburst
emission overshines the LSB component out to a galactocentric
radius of typically 2 exponential scale lengths.
Only at larger radii, where the starburst emission is in most cases
negligible, the LSB host can be studied directly.
However, to explore its possible dynamical influence on the global SF
process, it is essential to pin down its intensity distribution at
smaller radii, if possible just beneath the SF regions. Deprojection
of SBPs would then allow one to put constraints on the density
profile and the gravitational potential of the evolved stellar
host (cf., e.g., P96a). In addition, one would be able
to correct optical colors inside the SF component
for the line-of-sight contribution of the underlying stellar background
(cf. Cairós et al. 2002; Papaderos et al. 2002), thus better constrain the SF history of these systems.
One way to alleviate the problem of the extended starburst emission
is to extend studies to the Near Infrared (NIR).
At these wavelengths, the young stellar populations and the ionized gas contribute a smaller
fraction of the total light of the galaxy than in the optical.
For instance, evolutionary synthesis models by Krüger et al. (1995) predict
that a moderately strong burst during its peak luminosity accounts
for only 20% of a BCD's emission in the K, but for
80% in the B band.
Observations of BCDs also show that starburst emission is weaker
in the NIR, and becomes negligible at a smaller galactocentric
distance than in the optical
(e.g. Vanzi et al. 1996, 2002; Beck et al. 1997; Alton et al. 1994; James 1994).
NIR data allow therefore to study
the stellar LSB component at smaller radii and, using optical-NIR colors
(e.g. B-J), better constrain its formation history.
To achieve these objectives one needs to unambiguously detect the
LSB component and study its intensity over a sufficient span
(
mag).
Empirical estimates, based on published optical and NIR data, suggest
that these requirements are met by extending NIR studies to
22-24 J mag/
.
So far, only a few BCDs have been studied at
those surface brightness levels with an accuracy high enough to
pinpoint structural properties and colors (Mkn 86, Gil de Paz et al. 2000a; Tol 0645-376; Doublier et al. 2001; Tol3; Vanzi et al. 2002).
The present analysis is the first part of an observational project (see also Cairós et al. 2003; hereafter C03), aiming at a systematic study of the NIR properties of nearby BCDs with large array detectors on 4 m-class telescopes. For this purpose, we take advantage of a large set of imaging data, homogeneous with respect to its limiting surface brightness and the methods used in its processing. The observations have been conducted so as to permit NIR surface photometry out to a comparable radius as in optical wavelengths, with the purpose of a multiwavelength investigation of the LSB component.
This paper is structured as follows: In Sect. 2, we describe the sample selection and data acquisition, and discuss the data reduction, photometric transformations and extinction corrections. Section 3 focusses on the derivation of SBPs and their decomposition into the luminosity components owing to the old LSB host and the younger stellar populations and SF regions. In Sect. 4, individual objects are presented in detail. Our results are discussed in Sect. 5. Section 6 summarizes this work.
Our sample covers the whole morphological spectrum of BCDs, containing both
examples of the main morphological class (iE/nE systems according to
LT86), and of the less frequent iI/iI,C/i0 BCD classes. The
latter subset includes the metal-poor galaxies Tol 65 and
Tol 1214-277 (Z < /20). The sample also includes one
intrinsically luminous
blue compact galaxy (BCG), UM 448.
Table 1 lists the adopted distance to each BCD.
Whenever available, literature distances based on standard candles
were preferred for nearby objects. Those literature distances that
rely on redshifts have been corrected for peculiar motions within
the local supercluster and assume a Hubble constant H0= 75 km s-1 Mpc-1.
When no literature data was available, the distance was inferred from
the H0 and the heliocentric velocity listed in the
NASA Extragalactic Database (NED).
The latter was first transformed to the velocity relative to the
local group (LG) centroid using the NED velocity calculator, and
subsequently corrected for a LG infall towards the Virgo Cluster
center (
)
at 200 km s-1
(Tammann & Sandage 1985).
The NIR images were observed with the ESO 3.6 m NTT telescope, La
Silla/Chile, during three consecutive nights from April 21st to
24th, 2000. The seeing ranged from 0
4 to 1
0 FWHM and
the transparency variations were
1% during the first and third night,
and
2% during the second night.
Additional NIR images were taken at the 3.6 m telescope of the
German-Spanish Astronomical Center, Calar Alto, Spain, during several
observing runs. The seeing conditions were generally average (December 26th, 1999: FWHM 1
3-3
5, transparency fair; May
10th-15th, 2000: FWHM 0
8-1
4, transparency good to
average; October 6th-10th, 2000: FWHM 1
2-2
0,
transparency good to fair).
Both cameras used, the SOFI at the NTT Nasmyth focus and the OMEGA
PRIME at the Calar Alto 3.6 m telescope's prime focus, were equipped
with 1024
1024 pixel Rockwell HAWAII detectors. The pixel
scales for SOFI, using the large field objective, and OMEGA PRIME were
0
292 and 0
396, yielding a FOV of 4
94 and 6
76, respectively.
The data was taken through the J and H broad band filters, as well
as the modified K filters
at the NTT and K
at Calar
Alto, both selected to extenuate the contribution of thermal background.
To achieve an adequate sampling of the background variations, the telescope was offset between exposures typically each 60 s. While compact galaxies were jittered within the FOV, larger objects required to alternately observe the object and the sky. To avoid detector saturation, images were obtained by summing up subexposures of few seconds each. The total on-object exposure times, after rejection of subexposures affected by unstable readout electronics or strong background gradients, are listed in Table 1.
Table 1: Sample galaxies.
The exposures have been processed semi-interactively, employing our
NIR image reduction software based on ESO MIDAS. The procedures used for basic reduction, background
subtraction, image alignment and coaddition follow the recipes which
are detailed in, e.g., the SOFI User Manual
.
Since surface photometry in the LSB regime depends sensitively on the quality of the background subtraction, care has been exercised to eliminate residuals of this correction on both, small and large spatial scales. For this purpose we have implemented a number of additional corrective steps into our software package. For each single science frame of an exposure sequence we first computed an individual master background image, using subexposures that were taken closest in time, but at a sufficiently large distance from the target. Prior to the calculation of the master background frame, the input background images were cleaned from bright contaminating sources and normalized to the same mean intensity (see the SOFI User Manual for details). The master background frame computed this way was in turn scaled to the background level of the respective science frame, and subtracted. Each of the resulting subexposures was subsequently checked for a possible residual background gradient and, whenever necessary, rectified using a first order polynomial fit. The result image was obtained through combination of all background-corrected and coregistered subexposures. In this procedure, out of all pixels with identical sky coordinates, only those at sound detector coordinates have been included in the calculations.
Small residual background variations in the final images needed to be interactively corrected by fitting twodimensional polynomials. Alternatively, regions still affected by small-scale residuals in the background subtraction were extracted as subimages, corrected in the latter manner, and inserted back into the original frame. Likewise, bleeding artifacts, caused by bright sources in the NTT/SOFI FOV, or blooming of bright, overexposed foreground stars in the very outskirts of IC 4662 (northern edge) and Tol 1400-411 (east and west) were modeled and subtracted out. SBPs of the latter two galaxies were only analyzed well above intensity levels where slight residuals from the replaced bright stars could possibly contribute.
Final images taken in the same filter during different nights or at different telescopes were aligned to each other, transformed to equal pixel scales and resolutions, and coadded after weighting each one by its (S/N)2. The FWHM of the result images is listed in Table 1. Images used for aperture or surface photometry were manually cleaned for fore- and background sources.
The SOFI data was calibrated by observing at different airmasses
standard stars from Persson et al. (1998) six times each
night. The excellent photometric stability throughout the NTT
observing run has allowed for the derivation of an airmass-dependent
calibration with a scatter of 0.01 mag during nights 1 and 3 and
0.02 mag during night 2. Zero points and airmass-dependent
calibration coefficients both agree well with the average values
supplied by the NTT/SOFI support team.
Images of UM 448 and Mkn 1329, obtained by combining Calar Alto
and NTT data, were calibrated using aperture photometry of bright
stars in the SOFI FOV. Despite different K filters used at those
telescopes, the integral
fluxes are essentially preserved, as
color terms in the filter transformations (Eqs. (3)) are
not exceeding a few 0.01 mag.
The BCDs Haro 14 and Mkn 178, for which no NTT observations
are available, were calibrated using the
Two Micron All Sky Survey (2MASS) catalogue (Cutri et al. 2000; Jarrett et al. 2000).
As 2MASS data (cf. Andreon 2002) may, due to their
limited sensitivity, slightly underestimate the flux within the extended
LSB component, we computed calibration terms using 2MASS field stars in the close vicinity
of Haro 14 and Mkn 178.
Unless stated otherwise, all magnitudes and colors given in this paper refer to the calibrations described in the previous section, and to the photometric systems defined by the instrumental setup of each telescope (see Sect. 2). However, whenever photometric quantities of the sample BCDs are compared either among each other, or with model predictions and data from the literature, they are first transformed to the 2MASS photometric system, using the relations described below.
The calibration obtained at the NTT with the SOFI instrument is based
on standards by Persson et al. (1998), and can therefore
be transformed to the Persson et al. (1998) Las Campanas
Observatory (LCO) system using the color transformations given by the
SOFI user manual (Issue 1.3, 16/08/2000). A transformation from the
latter system to the 2MASS system is described by Carpenter
(2001). Combination of both latter transformations
yields the relations:
No transformations to other standard NIR systems are available
for the photometric system defined by the Calar Alto 3.6 m telescope
and the OMEGA PRIME camera. As stated in Sect. 2.4,
the calibration of the Calar Alto frames is tied to the 2MASS zero
points. The remaining uncertainties introduced by the unknown color
terms can be estimated from Carpenter (2001) to be
![]() |
(2) |
The K
and
magnitudes are related by the
following equations (cf. Wainscoat & Cowie 1992
and the SOFI user manual):
Magnitudes and colors given in this paper are corrected for Galactic extinction, adopting values derived from the B band extinction maps by Schlegel et al. (1998) (cf. Table 1) and the standard (RV=3.1) extinction law (Cardelli et al. 1989) implemented into the NED. No attempt was made to correct for internal extinction, since this is known to vary spatially even in the most metal-deficient BCDs (cf., e.g., Guseva et al. 2001; Cannon et al. 2002; Hunt et al. 2003) and can be reliably constrained in the SF regions only.
Surface photometry aims at a standardized one-dimensional
representation of a galaxy's two-dimensional flux pattern.
One technique to compute surface brightness profiles (SBPs)
requires the determination of the size
of the galaxy in
for a series of surface brightness levels
(mag/
).
By this definition, the equivalent radius R*=
is a monotonic function of the surface brightness
and vice versa.
In order to derive SBPs this way, one has to keep track of the morphology and
angular extent of a BCD throughout its intensity span, i.e. in general to be
able to interpolate an isophote down to the faintest measured level
of an SBP. By this condition one can visually check for and screen-out
fore- or background sources in the periphery of the galaxy, thus make sure
that source confusion does not affect SBPs at faint levels.
This task is more difficult to achieve when computing SBPs
employing photon statistics inside circular or elliptical
annuli, extending out to a user-defined maximal radius
.
SBPs derived for an irregular system by such techniques may vary
from case to case, depending on, e.g., the adopted
or
"center'' of the galaxy. It should be borne in mind
that techniques of this kind, when applied to galaxies with a
morphology significantly departing from the assumed circular
symmetry, can strongly overestimate the exponential scale
length and underestimate the central intensity of the LSB component
(cf., e.g., Marlowe et al. 1997).
In the present analysis, we compute SBPs using method iv),
described in Papaderos et al. (2002). This is a
hybrid technique, incorporating features of both aforementioned
approaches (determination of the R* corresponding to a user-defined
,
as opposed to the determination of the mean surface brightness
inside a circular annulus with a user-defined
radius R*
R*).
For a set of n intensity intervals In, with a mean intensity
decreasing as n increases, masks Mn are generated, each of which
extracts from a smoothed image of the galaxy the areas with intensities
within In. The equivalent radius
corresponding to the
mask area An is:
As a check for consistency, we also derived SBPs through ellipse fitting to isophotes or methods ii) and iii) in P96a, applying in all cases a minimum of image filtering to moderate photon noise. The resulting SBPs have been compared with deep optical surface photometry to ensure that the underlying LSB component has been detected and modelled (see Sect. 3.2) over a sufficient radius range.
Color profiles were derived by subtracting the SBPs from each other, after the latter had been smoothed to equal FWHM. Surface brightness and color profiles, corrected for Galactic extinction (Sect. 2.4.2), are shown in Figs. 2 through 14.
The uncertainties of each data point include the fully propagated effects of (i) Poisson photon noise and (ii) residual background variations on various spatial scales. The latter originate both from small residuals from the background reduction, and from undetected fore- and background sources. The treatment of (i) is explained in, e.g., P96a or Cairós et al. (2001a). The resulting uncertainties are in most cases small, owing to the large number of pixels typically involved in the calculation of each profile point at small S/Nlevels, i.e. in the outer LSB region of a galaxy. In these low S/N regions, the uncertainties from (ii) are typically dominant: background variations on spatial scales which are not small compared to the size of the considered mask segment or aperture do not average out.
The amplitude of such variations was estimated as the standard
deviation of the mean intensities measured in quadratic apertures,
placed on the background well away from the object, with sizes
of the order 1/10 of the object's diameter.
The resulting uncertainties of each SBP point might be decreased by a
factor of
,
accounting for the averaging of
background variations over the area An (Eq. (4)) of the
respective mask segment. This first-order correction was however
discarded, since the background variations cannot be determined at the
position of the galaxy's LSB component, and since the outer mask
segments cut through large areas of the image, possibly sampling
individual stronger background anomalies. Error bars shown in the
SBPs and color profiles (Figs. 2 through 14)
therefore represent upper limits, i.e. typically overestimates, to the
true errors, which are difficult to quantify. Nevertheless, the error
constraints shown here give a better estimate of the true errors than
the, typically too small, pure Poisson noise errors.
The reliability limits of the SBPs were not estimated from these error constraints, but determined at a surface brightness where an isophote could still be robustly interpolated on a mildly smoothed image.
The stellar emission of a BCD is due to the superposition of two
distinct populations with respect to their
ratio and
spatial extent: (i) the underlying LSB host galaxy which, owing to
its high
,
contains the bulk of the system's stellar
mass, and (ii) the younger stellar population, attributable to the
ongoing and recent SF activity. In the main class of iE/nE BCDs, the
younger stellar component dominates the optical emission inside
2
exponential scale lengths of the underlying LSB host and contributes,
together with ionized gas emission,
1/2 of the B light within
the 25 B mag/
isophote (P96b, Noeske
1999; Salzer & Norton 1999).
Evidently, a meaningful study of the structural properties
of a BCD requires the decomposition of SBPs into these
two main photometric components.
In the following, we employ a simple SBP decomposition scheme,
in which we fit only the LSB emission. Subtraction of the best fit
LSB model from the SBP allows us to deduce the luminosity fraction
and spatial extent of the superimposed SF component.
In order to make sure that extended starburst emission does not
affect the decomposition results, we fitted the LSB component
beyond a
transition radius
(cf. P96a, C03),
where optical and optical-NIR color
gradients vanish and isophotes become more regular. In addition,
whenever available, we used H
maps to trace the size of the SF
component, and get from it an additional constraint to
.
The LSB
emission has been fit out to the radius where SBPs became increasingly
uncertain as a result of background noise (typically for
0.5
for method iv). The fitted SBP range in
J is indicated with the dashed-gray line at the bottom of each
profile (Figs. 2-14).
An exponential fitting law
However, for several of our sample BCDs, an extrapolation of the
exponential LSB slope to smaller radii yields for R*
1...
3
a higher intensity than the observed value. Thus, a
meaningful decomposition of such SBPs cannot be achieved on the usual
assumption that the exponential law is valid in the LSB component all
the way to R* = 0
.
Instead, one has to adopt an alternative
fitting formula, approaching the exponential law for large radii and
flattening in its inner part. Such a distribution should be compatible
to the "type II'' profiles of disc galaxies (Freeman
1970, see also MacArthur et al. 2003), or
the "type V'' profiles described for spheroidal early type DGs by
Binggeli & Cameron (1991).
Previous optical studies have allowed for the detection and modelling of such an inwards flattening exponential LSB profile in a few BCDs only (e.g. I Zw 115, P96a; Tol 65, Papaderos et al. 1999, hereafter P99; Tol 1214-277, Fricke et al. 2001, hereafter F01; SBS 0940+544, Guseva et al. 2001). However, SBPs of this kind do not appear to be rare among intrinsically faint dEs and dIs. They have been observed in several dEs with and without a central nucleus (cf. Binggeli & Cameron 1991; Cellone et al. 1994; Young & Currie 1994, 1995), and Vennik et al. (2000) deduce a fraction of >10% for the SBPs of late-type dwarf galaxies falling into this category. In fact, a casual inspection of the sample of Patterson & Thuan (1996), Makarova et al. (1998), Vennik et al. (2000) and van Zee (2000) reveals several examples of dIs showing an exponential outer LSB slope and a pronounced flattening for intermediate to small radii (e.g., UGC 2034, UGC 2053, UGC 5423, UGC 9128, UGC 10669 in the sample of Patterson & Thuan 1996, KKH35 and KKSG 19 in Makarova et al. 1998, or UGCA 9, UGCA 15, UGC 2345, UGC 9240, UGC 10445 in the sample by van Zee 2000). Other examples are the dIs Holmberg I (Ott et al. 2001), Holmberg II (Noeske 1999), H 1032-2722 (Duc et al. 1999) and Kar 50 (Davidge 2002). Following the nomenclature of Binggeli & Cameron (1991) we shall henceforth refer to this type of SBPs as to type V.
Table 2: Structural properties of the dwarfsa; see also the discussion of individual objects.
The physical origin and exact form of type V profiles in puffed-up
stellar systems has not yet been studied in detail, neither
observationally nor theoretically. Therefore it is difficult to say
which fitting formula approximates best their intensity distribution
(see discussion in Sect. 5.3). An empirical fitting function
that yields by deprojection a finite luminosity density for R* = 0
has been proposed in P96a, as
![]() |
(7) |
In order to deduce plausible constraints to (b, q), we first
subtracted from the J image of each sample BCD most of the irregular
starburst emission. The latter has been approximated by an unsharp-mask
version of the original image (Sect. 3.4), adjusted such that no
regular emission from the LSB component was removed.
H
exposures and color maps were used to further ascertain that the
subtracted emission was not part of the underlying host galaxy.
J band exposures processed this way were then used to compute
SBPs for the LSB component.
These profiles, denoted
,
allow to better trace the LSB
component down to smaller R* than the ones derived prior to partial
subtraction of the starburst light.
By fitting Eq. (6) to the
SBPs we derived b, q(first column in Table 2). In most cases, fit
uncertainties are 0.1 and 0.05 for b and q, respectively. No
reliable
profiles could be computed for Tol 1214-277. For this system we fixed (b, q) to values inferred by
F01 from optical VLT data.
Alternatively, type V
profiles were fit with
a Sérsic model (Sérsic 1968) of the form
Note that Eq. (8) approximates well the projected light of
a variety of stellar populations with both a nearly constant
(ellipticals or bulges; Caon et al. 1993;
Andredakis et al. 1995; Graham et al. 1996, early-type dwarfs; Cellone et al. 1994; Young & Currie 1994) or
strongly varying
(for instance, nE BCDs or the plateau component
in the SBP of iE BCDs; P96a).
Therefore a Sérsic exponent
gives no strong indication for a stellar
system being similar to an intrinsically luminous early-type galaxy
(see also discussion in C03).
The Sérsicexponents of the LSB components (
)
were
obtained from non-weighted fits to the
SBPs (see above)
of type V profiles.
This limits
to values < 1.
We avoided to fit Eq. (8) to the outer part
of SBPs alone, i.e. for R*
,
as solutions obtained
this way are very uncertain and depend strongly on the accuracy
of the sky subtraction (see Sect. 5.3 and detailed
discussion in C03).
Notwithstanding the fact that Eqs. (6) and (8) give
comparably good fits in terms of
,
we decided not to
include the full (
,
,
)
Sérsicsolutions for the
SBPs in Table 2. Instead, in its Col. 1,
we quote only the Sérsic exponent
,
the uncertainties
of which are estimated to be of the order of 20%.
The photometric quantities of the sample BCDs are summarized in Table 2. BCDs without signatures of a type V profile in their
underlying LSB component
are marked with an asterisk in Col. 1.
For the remaining systems we list the (b, q) and
parameters, as
obtained respectively by fitting Eqs. (6) and (8) to
SBPs.
Columns 3 and 4 list, respectively, the extrapolated central surface brightness
(mag/
)
and exponential scale length (kpc), obtained
by fitting Eq. (5) to the outer exponential LSB part
of each SBP. Column 5 lists the total apparent magnitude of the LSB component, computed by extrapolating the fitted model
(i.e. Eq. (5) or Eq. (6)) to
.
Columns 6 through 9 list the radii and magnitudes of the star-forming
(P) and underlying stellar LSB component (E), as obtained by profile
decomposition. Following P96a, we measure the respective radial extent
(
,
)
and encircled magnitude (
,
)
of each component at an isophotal level
iso, taken to be 23 mag/
for J and 22 mag/
for H and K. The
isophotal radii determined for the sample BCDs at 23 J mag/
turn
out to be comparable to those obtained from optical SBPs at 25 B mag/
(P25 and E25 in P96a).
Column 10 lists the magnitude from an SBP integration out to
the last data point, and total magnitudes from aperture measurements
(cf. Sect. 3.3) are listed in Col. 11. The radii
and r80, enclosing 50% and 80% of the SBP's
flux are included in Col. 12.
Finally, a formal Sérsicexponent for the whole SBP
(
), for later comparison with literature data,
is listed in Col. 13 of Table 2.
Errors of the fitting law parameters determined from unweighted fits are an underestimation of the true errors. LSB slope differences between J, H and K (Table 2) should therefore not be considered significant, but rather to reflect the true exponential slope uncertainties, which are typically up to 10% in J, and may become somewhat larger in H and K, depending on the data quality.
Since a SBP can only be accurately derived in regions with a
sufficiently high S/N (i.e., in general, down to the minimum
intensity level in which the morphology of a BCD can still be
visually checked and the problem of source confusion can be
handled; cf. Sect. 3.1), profile integration out to the
last measured data point may, in some cases, underestimate the
object's total flux.
For extended LSB sources, or when NIR SBPs do not go sufficiently
deep (e.g., the
SBP of Tol 1214-277), the fractional flux
missed can easily exceed 10%.
Similar problems may affect growth curve flux determinations,
since these basically rely on a crude SBP derivation and can
sensitively depend on the quality of background subtraction.
We therefore measured total magnitudes within polygonal apertures which extend typically out to 1.5 Holmberg radii (Col. 11 of Table 2), after removal of fore- and background sources from the area of interest (cf. Sect. 2.3). Errors take into account the Poisson noise and small-scale variations of the local background.
Magnitudes and colors of selected features, such as stellar clusters
or H II regions, have been corrected for the flux contribution
of the underlying LSB component, by interpolating the mean surface
brightness of adjacent regions. Values computed this way are marked
with the superscript .
As pointed out in Cairós et al. (2002) and Papaderos et al. (2002), corrections
for the LSB emission are generally
not negligible in optical wavelengths. That this statement is also true in
the NIR domain is illustrated on the example of the iE BCD UM 461
(Fig. 1); correction for the flux contribution of the
LSB background shifts even the brightest SF region (a,
17.26
mag, open circle) of this system by +0.25 mag and -0.1 mag in the J-H
and
color diagram.
A census and photometric study of compact stellar clusters
in the extended BCD sample included in this project
after correction for the LSB- and ionized gas emission
will be presented in a subsequent paper.
![]() |
Figure 1:
NIR two-color diagram for the brightest compact regions
in the iI BCD UM 461.
Filled circles show the colors of the regions a
through g indicated in Fig. 5, after correction
for the flux contribution of the underlying LSB host galaxy.
Open circles, connected with dotted lines, indicate the
color of the respective region prior to that correction.
The star marks the color of the stellar host galaxy ("LSB'').
The color range covered by red supergiants in the SMC
(Elias et al. 1985) is shown by the box labelled RSG
to the lower right. The temporal evolution of the J-H vs.
![]() ![]() ![]() |
Open with DEXTER |
The considerable intensity range of a BCD, from its faint LSB outskirts to the brightest nuclear starburst region, renders the detection of fine coherent morphological features in the central portion of the galaxy difficult. We use therefore a modified unsharp masking technique (cf., e.g., Papaderos 1998), referred to in the following as hierarchical binning (hb) - transformation. This contrast-enhancing procedure is stable against noise at low intensities and allows for the flux determination of faint sources within the bright background of a BCD, with an angular size smaller than a user-defined value. Morphological features of interest revealed using this procedure are displayed in the grayscale/isophote insets of the sample galaxies, and described in Sect. 4.
![]() |
Figure 2:
a) Contours overlaid with a J image of Tol
3 (D= 13.8 Mpc). North is up, east to the left. Contours, corrected for Galactic
extinction, go from 19 to 23.5 J mag/
![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
Colors of the LSB host galaxy were derived
as the error-weighted mean of the color profiles
for R* >
,
after rejection of deviant points, being probably
affected by uncertainties in the sky determination, and local
residuals in the subtraction of background sources.
For Tol 1400-411, Pox 4, Mkn 178 and IC 4662, uncertainties in the LSB
colors are larger, due to extended starburst emission or crowding with
nearby bright stars.
As for the very metal-deficient systems Tol 65 and Tol 1214-277,
the faintness of their LSB component in
has not permitted,
despite generous exposure times, to pin down their
colors.
Whenever calibrated optical data were available, optical-NIR colors
were derived. Because the quality of the SBPs was typically better in
J than in H, and the J-H color shows little evolution with time (few
0.1 mag) for old (1 Gyr) stellar populations, we derived
B-J colors instead of the more commonly used B-H colors.
The mean colors of the host galaxies are shown at the right edge of each color profile (Figs. 2-14), in the photometric system in which the respective galaxy was observed and calibrated (see Sect. 2.4.1). Table 3 lists the NIR colors, transformed to the 2MASS photometric system to facilitate comparisons and the B-J color, where available. Errors give cumulative uncertainties in the calibration, transformation to the 2MASS system, and the scatter and systematic uncertainty of each color profile.
Table 3: Colors of the host galaxya.
![]() |
Figure 3:
Top: J-H color map of the central region of Tol 3,
corrected for Galactic extinction. The overlaid J contours are
computed from the contrast-enhanced blow-up included in
Fig. 2 (see text for details). Bottom: EW(H![]() |
Open with DEXTER |
This luminous (
-18.0; Marlowe et al. 1999) and
relatively metal-rich (
/6...
/3; Kobulnicky et al. 1999; Schaerer et al. 1999; Marlowe et al. 1999 and references therein) BCD is known to be a member
of the NGC 3175 galaxy group (García 1993).
NIR and optical images reveal two compact (
0.3 kpc)
high-surface brightness (HSB) regions:
the brighter northwestern knot A, roughly coinciding with
the geometrical center of the smooth LSB host galaxy, and the fainter
knot B, located
1 kpc southeast of A (see Fig. 2a).
Either knot is the locus of ongoing star formation, as witnessed by
the detection of Wolf-Rayet features (Kunth & Sargent 1981;
Vacca & Conti 1992; Schaerer et al. 1999)
and red supergiants from CO absorption studies (Campbell & Terlevich 1984).
The intense SF activity in Tol 3 is also reflected on relatively
blue optical colors (B-V= 0.24, V-I= 0.28; Marlowe
et al. 1997) in its nuclear region, as
well as on copious H
emission (
;
Marlowe et al. 1997).
Deep H
imaging by Marlowe et al. (1995) revealed a
bipolar outflow roughly perpendicular to the major axis of
the BCD, extending out to
2.8 kpc from its nuclear region.
Unsharp-masked NIR images reveal a complex morphology in the
HSB regime of the BCD (inset in Fig. 2a).
Regions A and B are immersed in an extended "S-shaped'' pattern,
0.9 kpc in length, ending at its NW tip with a curved
feature (N arc). There is some evidence for propagation
of SF activities, as both archival optical NTT and NIR data reveal
signatures of a younger age and stronger ionized gas emission
towards region B (cf. Fig. 3, bottom).
For region A we determine within a rectangular 4
4
aperture colors of J-H= 0.61 (
0.56) and
H-K= 0.41 (
0.58). For knot B we infer
a J-H= 0.39 (
0.14) and H-K= 0.48 (
0.71).
Such colors
suggest a younger stellar age together with an appreciable ionized gas
contribution towards the SE part of the SF component.
The latter is verified from the
H
equivalent width (EW) map in Fig. 3 (bottom)
(see also Gil de Paz et al. 2002) which shows that
SE of knot B, and all over an extended rim perpendicular to the
major axis of the BCD, the EW(H
)
rises to >200 Å.
Interestingly, unsharp-masked NIR and optical images reveal on
larger scales a chain of faint knots
(depicted with crosses in the inset
of Fig. 2a), arranged over 2.5 kpc SW of regions A
and B. Their typical J
magnitudes of
20.6...19 mag
translate into absolute magnitudes of -10...-12 mag. The nature
and formation history of this extended feature is intriguing. One
interpretation is that it delineates the approaching side of an oblate
star-forming shell triggered by the burst, or that it may be
associated with an inclined circumnuclear disk of
1 kpc in
radius. The first hypothesis is consistent with the findings by Alton
et al. (1994), who suggested from imaging-polarimetry the
presence of a large-scale bipolar reflection dust-nebula illuminated
by the central starburst region. This could provide effective
UV-shielding, thereby allowing for secondary SF activity. The
systematically redder colors in the NE half of Tol 3
(Fig. 3, top) are also in line with the same hypothesis, if
they originate from the more strongly absorbed far side of the
galaxy.
![]() |
Figure 4:
Haro 14 (D = 12.5 Mpc). For explanations of symbols and labels, refer to
Fig. 2. a) J band image and isophotes. Note that
the center of the star-forming regions is offset by ![]() ![]() |
Open with DEXTER |
SBPs in the NIR (Fig. 2b) show in the radius range
22
48
an exponential intensity fall-off
with a scale length
kpc. This value is close to the Bscale length of
0.48 kpc, inferred for the LSB component by
Marlowe et al. (1997) within
30
.
By contrast,
our surface photometry does not appear to be compatible to that of
Kunth et al. (1988). These authors show optical SBPs out to
a radius R* = 80
,
by a factor of 1.6 larger than the study here
or in Doublier et al. (1999) and up to 2.7 times larger
than in Marlowe et al. (1997).
NIR SBPs show no evidence for a dominant R1/4 profile in Tol 3
(cf. Kunth et al. 1988, Doublier et al. 1999).
A de Vaucouleurs profile is neither supported by the Sersic index
we derive for the entire J profile.
The NIR color profiles (Fig. 2c) show minor gradients (<0.15 mag kpc-1)
and level off to J-H= 0.54 mag and H-K= 0.12 mag for R* > 20
.
These results are not compatible to Doublier et al. (2001), who report for
R* > 10
a roughly linear J-H color increase from
0.5 mag to
2.0 mag.
A good agreement is found with Vanzi et al. (2002), who derive for the
LSB component of Tol 3 colors of J-H= 0.6 mag and H-K= 0.25 mag.
This relatively metal-rich nE BCD (Z
/3,
Hunter & Hoffman 1999)
shows SF activity ontop a smooth, nearly circular stellar LSB host galaxy
(Fig. 4a). The intensity distribution of the latter
(Fig. 4b) is approximated best by a modified
exponential distribution (Eq. (6)), flattening for
R*
20
(see the detailed discussion in Sect. 5.2).
The central part of the BCD contains a massive complex of SF regions,
displaced by 0.5 kpc SW of the geometrical center of the outer
LSB isophotes. This SF component measures
1.4 kpc in diameter
in the J band (this paper) and
2 kpc in the H
line (Marlowe
et al. 1997). Its composition out of several individual
regions, reported by Doublier et al. (1999) from optical
images, is also evident from the contrast-enhanced NIR images
(Fig. 4a, upper-left inset), which reveal a
wealth of individual regions, spanning a range of
3 mag.
The upper-right inset of Fig. 4a reveals a chain of faint
knots with a length of
1.5 kpc (labeled "E loop''),
extending eastwards from the main SF complex. There is a hint for a
similar feature in the southwestern direction.
Published data (e.g., the H
image by Marlowe et al. 1997) do not allow to assess whether these
faint features may trace induced star formation
along supergiant shells, as might be hypothesized from their morphology.
For the two brightest regions, denoted a and b, we obtain
respective absolute magnitudes of = -14.7 mag and -13.3 mag, and effective radii <80 pc.
If the mean E(B-V) = 0.35 for Haro 14 (Hunter & Hoffman 1999)
applies to knots a and b, then their de-reddened colors would
be J-H
0.6 and H-Ks
0.2.
Such colors are reached at the earliest when the NIR emission becomes
dominated by red supergiants (
10
30 Myr),
and suggest no strong nebular line contamination.
This points against substantial ongoing star formation in regions a and b.
The large spatial extent of SF sources in the inner portion
of the BCD is further evidenced by the profile decomposition
(Fig. 4b), yielding a plateau radius P
1.4 kpc in the J band.
The optical and NIR colors of the LSB host galaxy indicate
an age of several Gyr, in agreement with previous estimates
by Marlowe et al. (1999).
![]() |
Figure 5: UM 461 (D= 14.3 Mpc). For explanations of symbols and labels, refer to Fig. 2. a) J band image and isophotes. Bright stellar assemblies in the central portion of the BCD are indicated. b), c) Surface brightness and color profiles. The thick grey line shows a fit to the LSB host galaxy using Eq. (6) with parameters b,q=2.3,0.85. |
Open with DEXTER |
Telles & Terlevich (1995) suggested that the iI BCD UM 461 forms together with UM 463, UM 465 and UM 462 a loose group of dwarf galaxies.
SF activity is confined to the whole northeastern part of the
galaxy, i.e. within the 21.5 J mag/
isophote, or on a spatial scale
of
0.9
0.7 kpc.
The two brightest SF regions a (mJ
= 17.3 mag; Fig. 5a)
and c (mJ
= 19.5 mag) in UM 461 are separated by 20
in the
velocity space. This difference is of the order of the intrinsic H I
velocity dispersion within UM 461 (
30
;
van Zee et al. 1998).
The available data allow us to resolve
a manifold of morphological features within the SF component, most
notably a chain of compact sources northwest of region c
(labelled e-g) with a typical MJ
-10.4 mag.
Their J-H
colors of 0.5-0.6 mag are consistent with
the interpretation by Méndez & Esteban (2000) that
they are extinguished SF regions, formed not earlier than 100 Myr ago.
From profile decomposition we infer the summed up luminosity fraction of
compact and diffuse SF sources to
37% of the J band light of UM 461.
UM 461 shows in its faint outskirts a slight asymmetry towards the SW direction.
The intensity profile of the LSB host can be approximated by a med model with
a scale length
0.21 kpc and a depression parameter
0.85 (Fig. 5b).
The J band scale length derived here is in excellent agreement with the value
derived in the optical by Telles et al. (1997).
Color profiles reflect the ongoing SF for small radii
(R*
2
), and approach mean values of J-H= 0.49 mag and
0.17 mag in the LSB periphery. Such colors, together with
the
1.9 mag determined from optical data, point
consistently to a relatively evolved stellar LSB background.
The NIR colors derived here do not appear to be compatible
with those by Doublier et al. (2001).
These authors find within the radius range
5
R*
10
the J-H color to increase
from 1.5 to
2.3 mag, whereas the H-K color shows a continuous
decrease from -0.5 mag to <-1 mag.
Also, the integrated J-H and H-K colors of 0.99 mag and -0.68 mag,
respectively, listed in Doublier et al. differ significantly
from the values of 0.47 mag and 0.2 mag derived in the present study.
![]() |
Figure 6: Henize 2-10 (D= 8.7 Mpc). For explanations of symbols and labels, refer to Fig. 2. a) J band image and isophotes. The inset shows an unsharp-masked version of the nuclear region of the BCD (region marked by the brackets in the main image). b), c) Surface brightness and color profiles. |
Open with DEXTER |
This relatively metal-rich iE BCD (/2.4...
;
Schaerer et al. 1999; Kobulnicky et al. 1999) is the
first extragalactic system in which the broad
He II
4686 line was detected.
The starburst nature of He 2-10 is evidenced by a chain of bright
Super-Star Clusters (SSCs) in its brightest western SF region W
(Conti & Vacca 1994), extended X-ray and H
emission (Hensler et al. 1997, Papaderos & Fricke 1998), as well
as a large bipolar outflow from the SF region (Papaderos & Fricke
1998), with an expansion velocity between
250 and
360
(Méndez & Esteban 1999; Johnson et al. 2000).
Unsharp masking (inset in Fig. 6a) reveals a wealth of morphological features in the nuclear region of He 2-10, most notably an extended feature protruding northwest of region W (labelled NW) and an arc-like chain of compact sources connecting the tip of region NW with the secondary SF knot E. Ground-based B-R maps by Papaderos & Fricke (1998) indicate that region NW and the concatenation of sources bending southwards of it are considerably bluer than the underlying LSB host galaxy.
Our SBPs (Fig. 6b) show an exponential intensity decrease
in the LSB component (R*
20
), with a J scale length
identical to that obtained previously from optical data (
kpc, Papaderos & Fricke 1998).
![]() |
Figure 7: Tol 1400-411 (D= 4.8 Mpc). For explanations of symbols and labels, refer to Fig. 2. a) J band image and isophotes. The insets show magnifications of the bright SW star-forming knot A as well as the region B extending northwestwards from the major axis. The bluer NE region C is marked. b), c) Surface brightness and color profiles. The thick grey line shows a fit to the host galaxy using Eq. (6) with b,q=3.0,0.82. |
Open with DEXTER |
This cometary iI BCD is a member of the Cen A group (van den Bergh
2000), situated at a projected distance of 7.2
(
0.6 Mpc) from Cen A. Intense SF activity is taking place mainly
at the SW part of its elongated stellar LSB host, where ground-based NIR
and archival HST/WFPC2 optical data (PI: P. Seitzer, G0-8601) reveal
three bright (
16.2) stellar clusters spread over 8
(
200 pc). The appreciable ionized gas contribution in the SF component
of Tol 1400-411 is reflected on the very blue V-I color of
-0.4 mag,
in the brightest cluster, A (cf. l.h.s. blow-up of
Fig. 7a) and the
large EW(H
)
250 Å determined for this system
by Masegosa et al. (1994).
Interferometric 21 cm line studies (Fritz 2000) show that
the BCD is immersed within a large, rotationally supported H I cloud of
about 8
16 kpc in size. The H I halo reveals two surface
density maxima, one coinciding with region A and the other one
located
1
to the east.
The present data, as well as H
images by Fritz (2000),
reveal signatures of low-level SF activity all over the 22 J mag/
isophote of the BCD, i.e. 2.6 kpc across. Interestingly, 30
NE
of region A our data show a nearly circular region,
12
in diameter, with relatively blue (B-R of
0.75,
0.45 mag) colors in its unresolved interior
(denoted B in Fig. 7a). Such colors, being significantly
bluer than those in the LSB host (
1, V-I= 0.6-0.7), suggest
that region B is comparatively young. This is also suggested by the
NIR colors (J-K= 0.7...0.9) and luminosities (MJ
= -8.6...-9.7)
of bright sources therein which are consistent with a population of young
(10-20 Myr) red supergiants (cf. e.g. Elias et al. 1985;
Bertelli et al. 1994).
Another conspicuous feature seen at the northeastern part of the BCD
is a comparatively blue (
)
curved strip
(region C), apparently bending from the NE tip of the
LSB component to the north; a counter-feature of this region
is probably present at the southern part of the BCD.
The large extent of the SF component, as well as several
bright foreground stars in the periphery of Tol 1400-411,
render the determination of the structural properties and
color of its LSB component difficult.
The SBPs (Fig. 7b) are approximated
best with Eq. (6) with (b, q) = (3.0,0.8).
The mean LSB colors, J-H= 0.42,
0.28 and V-I= 0.6-0.7
can be brought into rough agreement, given the uncertainties
discussed in Sect. 3.5.
The GALEV model (see Sect. 5.4) yields for a metal-poor stellar population
forming in a single burst or continuously an age between several
108 to a few 109 yr (cf. Fig. 17).
However, these colors might be influenced by ionized gas emission
and the younger stellar populations in the NE boundary.
![]() |
Figure 8:
Pox 4 and its companion Pox
4B (D= 46.7 Mpc). For explanations of symbols and labels, refer to Fig. 2.
a) J band image and isophotes. The inset shows an unsharp-masked
close-up of the SF regions in Pox 4 (area within
brackets in the main image). M3, M6 and M9 mark the brightest
H II regions referred to as 3, 6 and 9 by Méndez & Esteban
(1999). The ellipse of small boxes marks the ring of
SF regions described by the same authors.
In our data, M9 resolves into 3 knots arranged over a projected
length of 3
![]() |
Open with DEXTER |
Despite its moderate metal deficiency (/9.3...
/7.6;
Kunth & Sargent 1983; Vacca & Conti 1992),
Pox 4 is a relatively luminous (MB= -18.81) iI BCG
(Méndez & Esteban 1999).
It is accompanied by a faint SF dwarf galaxy, Pox 4 B, at a projected
distance of 5 kpc and a velocity difference of 130 km s-1.
On our NIR images, we do not detect any emission in between
Pox 4 and Pox 4 B down to an approximative surface brightness
level of 24 J mag/
(Fig. 8a).
The optical morphology of Pox 4 is dominated by SF regions and
extended ionized gas emission on a spatial scale of 5
2 kpc
(cf. Figs. 1 and 2 in Méndez & Esteban 1999).
A substantial color shift due to intense (EW(H
) = 1410 Å) nebular
line emission has been observed in the brightest assembly of SF
sources M9 (inset of Fig. 8a) for which
Méndez & Esteban (1999) report a blue (-0.8 mag)
U-B together with an extremely red (+0.7 mag) B-V color.
The NIR colors of region M9 (J-H
= 0.28,
= 0.47) are
also suggestive of strong ionized gas contamination.
The latter is also reflected on the color profiles (Fig. 8c),
approaching for R*
3
(0.7 kpc) colors as extreme
as J-H= 0.2 mag together with
0.6 mag.
The SF morphology in Pox 4 is intriguing. Unsharp masking reveals eastwards
of region M9 a ring-like distribution of compact sources with a projected
size of 3.2
1.5 kpc and a position angle of
124
.
A similar morphology has been reported from optical data by
Méndez & Esteban (1999), who interpreted Pox 4
as a Cartwheel-like galaxy, downscaled by 1-2 orders of magnitude.
The available deep NIR data allow us to detect at faint intensities
(22.5 J mag/
)
a smooth underlying LSB host galaxy. Its
profile can be approximated with Eq. (6) and a
depression parameter q= 0.9 (Fig. 8b). From profile
decomposition we infer the absolute J magnitude of this stellar host
to -18.2 mag, which translates to MB> -17.2 mag for a
B-J> 1. Thus, despite its high total luminosity, Pox 4 qualifies
by the absolute magnitude of its LSB host as a dwarf galaxy.
Our SBPs show some similarity to the uncalibrated profiles
in Telles et al. (1997) out to R*=15
.
However, the scale length derived here for 10
R*
16
for the LSB component (
3
8=0.86 kpc) is twice as
large as that in Telles et al. (1997).
This intrinsically faint (MJ= -16.2 mag) galaxy has been
suggested by Méndez & Esteban (1999) to have
triggered the starburst activity in Pox 4 through a face-on
collision. As evidenced by faint H emission
(Méndez & Esteban 1999), Pox 4 B still maintains a
mild SF activity. The latter is probably taking place in
three compact (
0
5) sources, discernible in the central
part of the galaxy. Pox 4 B shows little morphological distortions,
except for a slight eastward extension of its LSB component
(Fig. 8a).
![]() |
Figure 9:
Tol 65 (D= 34.2 Mpc).
For explanations of symbols and labels, refer to Fig. 2.
a) J band image and isophotes. The sources b, c and e
along the NE-chain of SF regions are marked, following the
notation of P99. The feature marked by an asterisk
is a red (
![]() ![]() |
Open with DEXTER |
The SBPs (Fig. 8d) of Pox 4 B show for
2
R*
5
an exponential fall-off with an
0.29 kpc, and a flattening for R* < 2
.
We have
verified that the latter intensity regime is not due to seeing. For
this purpose, we convolved artificial exponential 2D models with
the observed
and extrapolated central surface brightness
(
19.5 J mag/
)
with a Gaussian kernel
matching the PSF of the coadded images (
FWHM).
The resulting SBPs deviate only slightly from a pure
exponential, even when the FWHM is further degraded by a factor of 2, and can by no means reproduce the strong flattening we detect in
the SBPs of Pox 4B. In addition, a perfect exponential distribution
with the
and
quoted above would yield a 16%
larger luminosity than the one actually measured for Pox 4B.
The available data do not allow us to study the NIR colors in spatial
detail. The mean J-H and
of respectively
0.4 and
0.2 are comparable to those of the LSB component of Pox 4.
Star formation in this very metal-deficient BCD (/24; Kunth &
Sargent 1983; Masegosa et al. 1994; Izotov et al. 2001) is taking place mainly in a chain of five compact
(1
2
2
)
sources located at the NE part of
an irregular, blue LSB envelope (P99).
The combination of an extraordinarily blue J-H
(
0.1 mag)
with a red H-K
(
0.8 mag) color in the surroundings
of the brightest SF region e (Fig. 9a) points
to a substantial contribution of ionized gas emission.
This is also suggested by the combination of a blue U-B (
-1.05 mag)
with a moderately red B-R (
0.4-0.5 mag) color derived by
P99 all over the NE half of Tol 65,
several 100 pc away from the opposite tips of the SF chain.
Further evidence for intense ionized gas emission is provided by Keck
spectroscopy by Izotov et al. (2001), who derived a
large EW(H
)
of >1000 Å in the SF component of the BCD.
Ionized gas contamination appears to be small midway in the SF
chain. The B-R= 0.15 mag (P99),
together with J-H
and
of 0.44 and 0.2 mag, respectively,
observed in region c are consistent with a single burst stellar
age of
100 Myr. Note that the J-H color in region c is
barely bluer than the average value for the LSB component (0.48 mag).
This is also true for the
color, which could be constrained
at
P
to
0.2 mag (cf. Fig. 9c).
Deep optical surface photometry (
28.5 B mag/
)
for Tol 65 has first
been presented in P99. These authors found optical SBPs
to show an outer exponential regime for R*> 7
and a
flattening relative to the exponential fit inwards of
R*
3
.
In order to adequately decompose
the optical SBPs, they modelled the LSB component with
Eq. (6) and a depression parameter
.
The type V profile of the LSB component is better visible
in NIR wavelengths, where an even stronger flattening is
required (
0.9) to fit the data.
The J SBP shows in the radius range
an exponential intensity fall-off with a scale length
kpc,
in good agreement with the value derived in P99
(
kpc).
The average LSB colors of J-H= 0.48
0.17, together with
a B-R
mag and
1.3 mag, inferable from
the SBPs in P99, are consistent with a stellar age
1 Gyr, assuming an instantaneous SF process
(cf. Sect. 3.5).
![]() |
Figure 10: Tol 1214-277 (D= 102.6 Mpc). For explanations of symbols and labels, refer to Fig. 2. a) J band image and isophotes. The bright star-forming complex at the NE edge of the galaxy ( a) is marked, along with the faint galaxies G1-G3 (see F01). b), c) Surface brightness and color profiles. The thick grey line shows a decomposition fit to the host galaxy by means of Eq. (6) with b,q=3.3,0.92. |
Open with DEXTER |
This metal-poor (/25; F01, Izotov et al. 2001) cometary iI BCD undergoes strong SF activity at
the NE end of an elongated stellar LSB body with an apparent size of
6.9
3.4 kpc. The brightest SF complex (labelled a in Fig. 10a) contributes nearly one half of the BCD's
optical light within the 25 B mag/
isophote (F01). The
J
magnitude determined for region a, 18.6 mag, shows that
in the NIR the starburst still provides
1/3 of the galaxy's
total flux. The colors of this region (J-H
= -0.02 mag and
= 0.51 mag) are consistent with a young burst age of <7 Myr
for the BCD (see also F01).
Only the J band SBP of Tol 1214-277 could be observed with a
sufficient quality for a decomposition (Fig. 10b).
Similar to Tol 65, it shows a type V distribution, as already reported from
deep optical VLT FORS I data by F01.
Fitting Eq. (6) to the
SBP, with the (b, q) parameters
fixed to the values derived in F01 (3.3,0.92), we obtain a scale length
0.53 kpc, slightly larger than that in the optical
(
0.49 kpc; F01).
Fricke et al. (2001) reported for Tol 1214-277 nearly constant U-B and B-Rcolors of respectively -0.42 mag and 0.34 mag over an intensity span of 8 mag (colors
adapted to the galactic absorption assumed here).
Such colors, together with the
0.4 mag and
1.1 mag
derived here, are slightly bluer than those of Tol 65, supporting the hypothesis
that Tol 1214-277 is a relatively unevolved dwarf galaxy.
![]() |
Figure 11: Mkn 178 (D= 4.2 Mpc). For explanations of symbols and labels, refer to Fig. 2. a) J band image and isophotes. The inset shows a close-up of the star-forming regions (delimited by brackets in the main image). The regions a-d described by Gonzalez-Riestra et al. (1988) and Papaderos et al. (2002) are labeled. b), c) Surface brightness and color profiles. |
Open with DEXTER |
The spectroscopic properties of this intrinsically faint (MB= -13.9 mag) iE BCD, a member of the CVn cloud I of galaxies (Makarova et al. 1998), have been investigated in
e.g. González-Riestra et al. (1988) and Guseva et al. (2000).
Optical images reveal a complex morphology in the SF component,
notably a pronounced separation of HSB regions into a compact assembly
of bright knots to the SE (regions a and b) and an
extended arc-like segregation of fainter sources to the NE. NIR
images, on the contrary, show a more regular morphology, with only a
few prominent sources in the central part of Mkn 178 (see
Fig. 11a).
Region a (mJ = 17.5 mag; cf. inset in Fig. 11a)
coincides with the optically brightest region in the BCD and is the
main locus of active star formation. The brightest NIR source, b
(mJ
= 16.5 mag), is optically faint and shows nearly the same B-Rcolor as the surrounding diffuse emission within the plateau component
(cf. Papaderos et al. 2002).
Sources c and d are immersed within the northern
featureless region. The non-detection of an optical counterpart for
region d, and the overall optical/NIR morphology of the SF component
are suggestive of inhomogeneous, large-scale dust absorption on
a spatial scale of
1 kpc. This may cause the apparent
separation of the SF component in two large detached complexes
and hide knot d in optical wavelengths.
González-Riestra et al. (1988) and Guseva et al. (2000) have shown ionized gas emission to be negligible
in region b (EW(H) = 24...34 Å). By the color
excess of E(B-V) = 0.25 given in González-Riestra et al. (1988), its observed B-R color (0.67 mag, Papaderos
et al. 2002) transforms to 0.28 mag, suggesting a
single-burst age of
100 Myr. This is consistent with the colors
of J-H
= 0.24, H-K'
= 0.06 and B-J
= 1.6 mag obtained in the
present work, if they are de-reddened adopting the same amount of
intrinsic extinction.
The presence of stellar complexes with an age of the order of
108 yr is in line with the detection of numerous
luminous AGB stars in NIR color-magnitude diagrams
(Schulte-Ladbeck et al. 2000), from which
these authors infer significant SF activity over the
last few 108 yr.
NIR SBPs of Mkn 178 (Fig. 11b) resemble closely
the optical ones (Papaderos et al. 2002), showing an
extended plateau component ontop a smooth, exponential LSB host
galaxy. For the latter we derive a J band scale length of 0.28 kpc
and colors (
0.53,
0.2,
2.0
and
1.1 mag) that consistently indicate an old stellar LSB
background.
![]() |
Figure 12: Mkn 1329 (D= 16 Mpc). For explanations of symbols and labels, refer to Fig. 2. a) J band image and isophotes. inset: hb-transformed image of the central area (delimited by the brackets in the main image), showing the distribution of the bright regions along the major axis. The white cross marks the bright SW star-forming region for orientation. b), c) Surface brightness and color profiles. The thick grey line shows a decomposition fit to the host galaxy by means of a modified exponential distribution, Eq. (6), with b,q=1.6,0.70. |
Open with DEXTER |
This relatively metal-rich (/5, Guseva et al. 2000)
cometary iI BCD (
-16.8, Yasuda et al. 1997)
is associated with a group of 11 galaxies (LGG 296, García 1993)
within the Virgo Cluster.
As shown in the contrast-enhanced image (inset in
Fig. 12a), numerous irregular concentrations are present in
the inner zone of its elongated stellar LSB component, over a
projected length of 3.5 kpc. An H
exposure by
Gallagher & Hunter (1989) shows that SF activity is
almost exclusively confined to a bright H II region (marked
with a cross in Fig. 12a) at the SW end of the
high-surface brightness component. The H
flux derived
for that SF region by the latter authors yields, at the
distance adopted here, an H
luminosity of
1.5
1040 erg s-1 and a star formation rate
(SFR) of
0.1
yr-1. The dominant SF region in
Mkn 1329 is identifiable with a compact (
300 pc),
luminous (MJ = -14) NIR source; its colors, (J-H)
= 0.16 and
(
)
= 0.51, are probably significantly influenced by nebular
emission (EW(H
) = 276 Å, Guseva et al. 2000).
The morphology of the SF regions and the high luminosity of the SW complex in Mkn 1329 are typical among cometary iI BCDs. The similarity of these systems with respect to the morphology of their LSB and SF component suggests a comparable evolutionary state/history or common mode of star formation. Propagation of SF activities along those objects' major axes has been proposed and observationally supported by a number of recent studies (cf. Noeske et al. 2000 and references therein).
The SBPs of Mkn 1329 (Fig. 12b) indicate a moderate central
flattening of the LSB component, which can be approximated both by a
Sérsic law with
,
or a med with
b,q = (1.6,0.7) (see Table 2 and
Sect. 3.2). The detection of a moderate central
flattening in the LSB component owes to the comparatively small
contribution of the superposed younger stellar population in Mkn 1329
(
17% of the total J band light).
The NIR colors in the outskirts of Mkn 1329, (J-H)
= 0.64, (
)
= 0.06,
suggest an evolved stellar LSB component. Adopting a B-J color characteristic
of a few Gyr old stellar population, we can estimate the B band structural
parameters from the
,
and
determined
from J SBPs. We find that the host galaxy of Mkn 1329 shows, similar to other
cometary BCDs (Noeske et al. 2000), structural properties intermediate
between extended dIs/dEs and compact iE/nE BCDs.
![]() |
Figure 13:
IC 4662 (D= 2 Mpc). For explanations of symbols and labels, refer to Fig. 2.
a)
J band image and isophotes. The insets show magnified
views of the star-forming region A in the central part of the BCD
and of the D component, about 1
![]() ![]() ![]() ![]() |
Open with DEXTER |
This nearby (D= 2 Mpc; Heydari-Malayeri et al. 1990) dwarf
galaxy has been studied spectroscopically by e.g. Pastoriza & Dottori
(1981), Stasinska et al. (1986),
Heydari-Malayeri et al. (1990) and Hidalgo-Gámez et al. (2001). The latter authors derived the oxygen
abundance in the brighter SF regions A&B to /6.5...
/7.6,
respectively. A continuum-subtracted H
map (Fig. 13b)
from Papaderos et al. (2002) reveals an extended and
complex morphology of the ionized gas emission in the upper half of
the galaxy, where the H
emission peaks, and shows a number of
shells extending up to
0.5 kpc NE. Another interesting feature
is the extranuclear region D, seen in the outer regions (
22 J mag/
)
of IC 4662. Hidalgo-Gámez et al. (2001) find
this H
emitting region to be less metal-rich than the central
SF regions (A), and to show a significant recession velocity
difference (250
150 km s-1, Hidalgo-Gámez et al. 2002) to IC 4662. Our deep images show that region D
is not well detached from the main body of diffuse H
emission in
IC4662 (contrary to Heydari-Malayeri et al. 1990), but
apparently connecting with regions A&B through a chain of H
sources.
Hidalgo-Gámez et al. (2002) have proposed that region D may be either a chemically and kinematically distinct complex within IC 4662, or a close companion object. The latter possibility is particularly interesting in view of the hypothesis that very close, gas-rich dwarf companions might be conceivable triggering agents of starburst activity in BCDs (Taylor et al. 1995; Pustilnik et al. 2001; Noeske et al. 2001).
From the available data, we resolve a wealth of morphological information in the central part (Fig. 13a) of IC 4662, notably the head-tail morphology of the starburst. Luminosities and colors of the brightest point sources in region A are in agreement with those of red supergiants, supporting the results by Heydari-Malayeri et al. (1990), as well as of blue supergiants and AGB stars (cf. the upper left inset in Fig. 13a; see Schulte-Ladbeck et al. 2001 for color limits separating the latter classes of giant stars).
As IC 4662 is located at low galactic latitude (-17.8
),
photometric studies of its LSB component are complicated by the dense
foreground Galactic stellar field. Also residuals in the removal of the NW bright star
may affect the photometry for faint isophotal levels.
A tentative exponential fit to the J SBP
yields for R*> 65
,
i.e. outside significant nebular emission, a scale length of
0.21 kpc. However, inspection of the
profile
shows that the BCD follows a type V distribution, fitted best with
Eq. (6) with a central surface brightness of 16.8 J mag/
,
a
scale length of
150 pc and a depression parameter as large as
.
If so, the starburst contributes
50% of the
total J light of the BCD. IC 4662 was hitherto classified
as a dwarf irregular galaxy (cf. e.g. de Vaucouleurs et al. 1991). However, the structural properties of its LSB
host, as well as its intense and spatially extended SF activity, place
IC 4662 in the range of BCDs, making it probably one of the closest BCDs
known.
![]() |
Figure 14: UM 448 (D= 76.1 Mpc). For explanations of symbols and labels, refer to Fig. 2. a) J band image and isophotes. The inset shows a contrast-enhanced close-up of the star-forming regions (delimited by brackets in the main image); individual knots ( a-c) are labeled. b), c) Surface brightness and color profiles. |
Open with DEXTER |
This is a distant (
76 Mpc, Mirabel & Sanders
1988) blue compact galaxy, known to be relatively
metal-deficient (
/8.5; Masegosa et al. 1994; Izotov
& Thuan 1999). Despite its resemblance
(Fig. 14a) to cometary iI BCDs, UM 448 does not
qualify as a dwarf due to its intrinsic luminosity (MB= -19.7 mag)
and large linear extent (major axis length
25 kpc). Active star
formation is taking place within an extended (diameter
7 kpc)
high surface brightness region at the NE part of the galaxy, showing
some fainter SW extension. The starburst component contributes
70% of the J band light. Guseva et al. (2000)
estimated the starburst luminosity to be powered by
O stars (value referring to the distance assumed here),
0.6% of which are undergoing their Wolf-Rayet phase.
Studies by Sage et al. (1992) indicate an ongoing SFR
of 21
yr-1, and a large reservoir of molecular gas
amounting to
or
1/2 of the
H I mass (
;
Mirabel & Sanders
1988) of UM 448.
On unsharp-masked images, the SF region splits
into 3 high-surface brightness entities (see the inset in
Fig. 14a), the brightest one being the reddest
(J-H = 0.7 mag) whereas the fainter regions b and c are
slightly bluer (J-H
0.5 mag).
The SBPs (Fig. 14b) show an exponential decay for
R* > 15
with a J band scale length of 3.8 kpc, somewhat
smaller than the value
5.2 kpc (14
)
inferred in Telles et al. (1997).
The SBPs derived for the sample BCDs in Sect. 4 bear close resemblance to those typically inferred from optical broad-band data (cf., e.g., P96a, Telles et al. 1997; Marlowe et al. 1997; Doublier et al. 1997,1999; Cairós et al. 2001a). In most cases, SBPs show in their outer part an exponential intensity decrease and red, nearly constant colors. This outermost SBP part is attributable to the underlying LSB host galaxy which, except for a few rare cases, shows clear evidence for a several Gyr old stellar population. At intermediate and small radii the emission of a BCD is dominated by the younger stellar component. In the main class of iE BCDs (LT86) its luminosity output is reflected on two conspicuous SBP components.
1) At small radii (R*
100 pc), a feature commonly observed in
optical and NIR SBPs is a central intensity excess. This is seen in
e.g., the J SBP of Tol 3, Mkn 178, Mkn 1329, Haro 14 (our sample) or
Mkn 36 and UM 462 (C03). This component
is due to the brightest and typically youngest stellar assembly.
In most cases, this narrow innermost excess is due to one and the same region in
both optical and NIR wavelengths. An exception is presented by
Mkn 178, where the brightest region in the NIR is offset from that
in the optical by
150 pc, or by the BCD Mkn 35 in the sample of C03.
2) The second SBP feature has been referred to as plateau in
P96a. It shows a nearly constant or slowly decaying
intensity, and extends typically out to
24 B mag/
.
SBPs with a prominent plateau on top a more extended exponential LSB envelope
can in general not be adequately fit by a simple function (e.g. a
Sérsicprofile) over their whole intensity span.
A meaningful decomposition of such SBPs is only possible when
an extra component (e.g., a Sérsicdistribution with
)
is introduced in order to fit the plateau
(P96a, Cairós et al. 2001a).
There is, so far, no observational support for the plateau being a
dynamically distinct or even interaction-induced stellar entity with
nearly constant
.
Quite contrary, appreciable color gradients (
1.5 B-R mag kpc-1)
in the HSB part of BCDs indicate that the plateau light is
mainly due to a young and moderately evolved stellar population.
P96b have shown that a conspicuous
plateau in the SBPs of many iE BCDs can naturally result
from the superposition of diffuse and compact SF sources with varying luminosity and
galactocentric distance on a more extended, exponential LSB component.
Conversely, the plateau is nearly absent in the intrinsically brighter
nE BCDs, where most of the starburst light originates from the nuclear
region of the BCD. The overall SBPs of these systems show frequently a concave
shape, fitted satisfactorily by Eq. (8) with a Sérsicexponent
1, and sometimes resemble a de Vaucouleurs profile.
Discerning the formation history of the stellar populations memorized
in the plateau light poses a challenge for surface photometry and CMD
studies, as nebular line emission (e.g. Tol 65; P99; Izotov et al. 2001, Tol 1214-277;
F01) and patchy dust absorption (Tol 3, Mkn 178; this
paper, Mkn 33 and Mkn 35; C03, I Zw 18; Cannon et al. 2002) may both hamper standard age-dating techniques.
In addition, an important and mostly overlooked source of systematic
uncertainties in the determination of colors or EWs within the plateau
stems from the unknown line-of-sight contribution of the underlying
old LSB component. As pointed out in Sect. 3.3, the latter
may redden colors of compact stellar clusters in the SF region by up
to a few tenths of magnitude. Corrections of this order have also been
reported for the iE BCDs Mkn 370 and Mkn 178 by Cairós et al. (2002) and Papaderos et al. (2002),
respectively.
Evidently, for such a correction one has to assume a model for
the intensity distribution of the LSB component just beneath the
SF regions (i.e., for R*
P
). It is a common practice to
extrapolate the exponential slope of the LSB periphery of BCDs
all the way to R* = 0
.
However, the universal validity
of this procedure for BCDs has been questioned in, e.g., P96a.
These authors discussed observational evidence for type V profiles
in BCDs and proposed the alternative fitting formula Eq. (6) to
approximate such convex distributions. Alternatively, C03 discuss
cases where the LSB host galaxy shows a concave profile which is better
fit by a Sérsicmodel with an exponent
.
The photometric structure of the LSB host of BCDs will be
discussed in the light of the present NIR data in the next section.
Nine of the BCDs included in our sample (Table 2) show
signatures of a type V profile in their LSB hosts. This is because an
exponential law, or any Sérsicdistribution with ,
fitted to
the outer part of their J SBPs, predicts at intermediate to small
radii a higher intensity than the observed value (cf. Sect. 3.2).
Of course, pure exponential fits to
profiles cannot always be
definitely ruled out within the 1
uncertainties.
However, such fits would either overestimate the central intensity of the
stellar LSB host, hence underestimate the luminosity fraction of the SF
component (see below) or systematically overestimate the
J band exponential scale length, thus imply an implausibly large B-J
color gradient for the old underlying LSB population.
The limited size of our present sample does hardly allow to estimate the
frequency of type V profiles in BCDs.
This issue will be addressed in a forthcoming paper of this series,
focussing on the complete NIR sample. However, the evidence gathered
so far (see also Sect. 3.2) strongly supports the idea
that a substantial fraction of BCDs shows type V profiles in its LSB host galaxy.
The rare detection of type V profiles in BCDs in previous optical studies
can be explained by the fact that
in those wavelengths extended starburst emission overshines the LSB host
within typically its inner 2 exponential scale lengths
(P96b, Noeske 1999; Cairós
2000; Cairós et al. 2001a).
As a result, a type V profile in iE/nE BCDs could be detected only when
a significant depression of the exponential LSB slope occurs for R*
,
i.e. if b>2 in Eq. (6).
NIR observations do not overcome, but alleviate the problem of severe light
pollution by the burst, proving in many cases crucial for disentangling a pure
exponential from a type V distribution in the underlying LSB galaxy.
![]() |
Figure 15: Decomposition of the B band SBP of Haro 14 (filled circles) assuming for its LSB host a pure exponential model (dotted line) or a modified exponential distribution (Eq. (6); thick gray curve) with the flattening parameters (b, q) = (3.6, 0.9) derived in the J band. The surface brightness distribution of the star-forming component, as obtained by subtraction of the respective LSB model from the B SBP is illustrated by the filled and open interconnected symbols (see discussion in the text). |
Open with DEXTER |
This is illustrated in Fig. 15 on the example of the
iE BCD Haro 14.
Its B SBP is roughly exponential out to R* = 40
(
27 B mag/
), suggesting that the exponential slope seen
in the LSB periphery continues all the way to R* = 0
.
In that case, one is tempted to conclude that the ongoing burst
gives rise to a modest luminosity increase, only.
Indeed, subtraction of the exponential LSB model from the
SBP suggests that SF sources (small interconnected squares in
Fig. 15) are all confined within R* = 8
and
that they account for no more than 7% (
16.5 mag) of the B light of the BCD. Evidently, this conclusion is hardly compatible
with the extended morphology of the SF component and copious H
emission of Haro 14 (cf. Marlowe et al. 1997; Doublier et al. 1999).
One arrives at a quite opposite conclusion on inspection of the
NIR profiles.
Figure 15 shows that a pure exponential law falls
short of fitting the J SBP inwards of R* < 20
;
a plausible
fit to the NIR data must invoke a significant flattening of the
exponential slope inside
3 scale lengths of the LSB component.
An adequate fit to the
is best achieved
with a med (Eq. (6)) with (b, q) = 3.6, 0.94.
Such a strongly flattened exponential LSB model yields
a stellar mass by a factor
3 smaller than that of a purely
exponential profile and implies that the starburst (small
interconnected circles in Fig. 15) contributes
60% of the B light within the Holmberg radius, i.e. more
than a factor of eight larger than the value inferred before.
The isophotal B magnitude within 25 B mag/
is then
increased by
1.1 mag due to the starburst.
This value is still compatible to the mean value of 0.75 mag deduced
for BCDs in P96b and Salzer & Norton (1999).
The considerations above show that the choice of the model for the LSB emission may have far-reaching implications to our view about the SF amplitude and photometric fading of BCDs. The usual assumption of a purely exponential LSB model may, in some cases, strongly underestimate the luminosity fraction of the young stellar population, and lead to the conclusion that a "burst'' is merely a minor luminosity enhancement in the lifetime of a BCD. If, on the contrary, the LSB emission is assumed to follow a type V profile, then the estimated luminosity of the superimposed young stellar population may increase by more than a magnitude.
Consequently, different LSB models imply different
amounts of photometric fading of a BCD once SF activities have terminated.
This has to be borne in mind when discussing evolutionary links
between BCDs and other dwarf galaxies, as well as a possible
fading of luminous blue compact galaxies at medium redshift to
local spheroidals (cf. e.g. Guzmán et al. 1998).
Note that the assumption of a type V profile has no effect on
the extrapolated central surface brightness
and the
exponential scale length
of the LSB host galaxy of a BCD.
As a result it is not expected
to significantly change the systematic difference between
BCDs and other types of dwarf galaxies on the
-
and
-
parameter space (P96b, Marlowe et al. 1997;
Salzer & Norton 1999; Papaderos et al. 2002).
A dedicated investigation of the frequency and origin of
systematic deviations from the exponential law
(e.g. a type V profile, or Sérsicprofile
with )
in the stellar LSB host of BCDs
is apparently of great interest.
A first step towards such studies is to test the suitability of
different empirical functions in approximating the observed LSB
intensity distribution.
This issue will be briefly discussed in the following section.
![]() |
Figure 16:
Upper panel: Fit to the LSB component of Haro 14
(thick gray curve, labelled 4) assuming an inwards flattened
exponential distribution ( med, Eq. (6)) with the
extrapolated central surface brightness
![]() ![]() ![]() ![]() |
Open with DEXTER |
As noted in Sect. 3.2, also the Sérsic law, commonly applied to structurally analyze various extragalactic systems, can approximate type V SBPs. In this section, we further discuss why in this work preference was given to the med model to fit and quantitatively study such SBPs.
Both the Sérsic law and the med are fitting functions of empirical
origin (Sect. 3.2). The choice of either model does
therefore not imply any assumptions on the physical background of
type V SBPs, such as the dynamics or stellar mass distribution of a galaxy.
However, although either function was found to satisfactorily
approximate the type V SBPs found in this work, they principally
differ with respect to their shape.
This difference is evident in the comparison shown in
Fig. 16, where we fit for illustrative purposes a Sérsic
profile (thin dashed line) to the med model (heavy line, labelled
4) obtained for the LSB component of Haro 14 in
Sect. 4. Distributions 2 and 3
illustrate med profiles with equal extrapolated central
surface brightness
(17.46 J mag/
)
and exponential
scale length
(0.37 kpc) as 4, but differing degrees of
flattening (q) with respect to the pure exponential (q=0) model 1.
The med 4 compares best to a Sérsicprofile with a central
surface brightness
J mag/
,
a scale length
1.14 kpc and an exponent
.
Note that the
deduced
compares well to the actual central
surface brightness of the LSB host galaxy
![]() |
(9) |
It is evident that the Sérsic law and the med distribution in
Fig. 16 closely follow each other over a radius range of
nearly 10 scale lengths .
Nevertheless, the lower panel of
Fig. 16 shows that the residuals between either model are
small (
0.2 mag), but systematic.
This difference reflects the fact that a Sérsic law with
differs from an exponential at all radii, while the med approaches an exponential slope for
.
We recall that both a Sérsic and a med model were found to
approximate well the inner portion of a med profile (e.g. within
1.5 cutoff-radii
). The question which distribution
gives a more adequate description of the observed type V SBPs can
hence only be assessed through studies of the outer parts of such
profiles. If the type V SBPs show a perfectly exponential fall-off in
their outer parts, continuing several scale lengths beyond a cutoff
radius
(cf. Eq. (6)), a Sérsic law will fail to correctly
reproduce these light distributions, and no Sérsicmodel fit to the LSB
emission can correctly recover the exponential scale length
and will not be free of ambiguity (Sect. 5.3.1).
Alternatively, if subsequent studies show that the LSB profiles
deviate systematically from the exponential law at all radii, then a
Sérsicprofile might be the preferable fitting law, albeit the drawbacks
discussed in the following Sect. 5.3.1.
Such considerations call for deeper photometric studies of the LSB
component of BCDs, and in dwarf galaxies in general. An ultimate
check requires deep surface photometry, allowing to derive SBPs out to
with an accuracy better than 0.2 mag (see the
residuals in Fig. 16), not reached by currently available
data.
As long as tests of this kind await to be done, we argue, in agreement with C03, that a pure (Eq. (5)) or modified exponential (Eq. (6)) fitting formula should be preferably used to fit the underlying LSB component of BCDs. Previous optical studies (e.g. LT86, P96a, Telles et al. 1997; Marlowe et al. 1997; Salzer & Norton 1999; Vennik et al. 2000; Cairós et al. 2001a, 2001b; Makarova et al. 2002) of BCDs suggest that those systems mostly show no strong deviations from an exponential law over their outer LSB SBPs. Also if small deviations from an exponential law which are hardly detectable in current data should exist, a med gives a robust approximation by fixing the LSB profile shape. While a Sérsiclaw at first view offers a greater flexibility in this profile region, its application in the extremely low S/N regime to quantify marginal deviations from an exponential is problematic, as discussed in the following section.
A drawback of Sérsicmodels when fitted to type V profiles is that
the solution can vary considerably, depending on the fitted radius
range.
For instance, a Sérsicfit to the med 4 (Fig. 16) for
<23.5 J mag/
,
or within 5
(
30
)
yields an
,
significantly lower than the value above. Note that a Sérsic
exponent
0.5 reflects a central minimum in the radial
luminosity density distribution of the LSB component (P96a). The
stability of the shape parameter
has been explored in detail in
C03; these authors remarked that a Sérsicfit can be very
uncertain for concave SBPs, where the exponent
can vary by up
to an order of magnitude, depending on the fitted range and the radial
sampling of points in the profile.
Another objection to the view that Sérsicmodels offer a robust tool to
systematize the structural properties of dwarfs comes from the
degeneracy of the shape parameter
vs. the pseudo-scale length
.
A fit of Eq. (8) to the med distribution 2
yields an
0.94 and a
0.44 kpc (>
). As
for profile 3, it is best approximated with an
and
kpc (
). We see that while the Sérsic
exponent
decreases with increasing degree of flattening,
i.e. as we go from distribution 2 towards 4, the
corresponding scale length
increases from 0.44 kpc to 1.15 kpc. Actually, the exponential scale length
of the med profiles considered here is nowhere recovered by fitting a Sérsic
model. Instead, one is left with the pseudo-scale length
,
which by itself alone carries no quantitative information on the
structural properties of the LSB component.
Obviously, the scale length
is only meaningful in connection
with
.
However, these two quantities are rendered impractical
for a systematic study of the LSB component by their strong non-linear
coupling (see discussion in e.g. Young & Currie 1994;
Cellone & Buzzoni 2001), and dependence on the fitting
procedure (see above). Furthermore, observational uncertainties
connected with, e.g., the sky subtraction (see e.g. Cellone & Buzzoni
2001, C03), filtering of images prior to surface
photometry, and imperfect removal of background sources may also skew
the
vs.
parameters in a hardly predictable manner,
making Sérsicfits to the outer part of SBPs a hazardous procedure
(C03). These problems are probably not worrisome in studies
of early type galaxies, i.e. systems with little morphological
distortions and a nearly constant
,
where also the
central, high S/N regions are accessible to constrain the global
photometric structure. In irregular SF galaxies, however, the derived
Sérsic parameters depend on subjective choices in the data processing
(see above), profile fitting and, equally important, on the profile
extraction methods themselves. This is particularly important in the
case of BCDs, where mostly only the outer, low S/N part of the LSB
host is accessible to structural studies.
It is conceivable that the latter circumstances imprint the Sérsic
parameters (
,
,
)
inferred recently for a
sample of luminous BCGs by Bergvall & Östlin (2002).
We believe that the extremely large
exponents they
derive (up to 20), in conjunction with central surface brightnesses
0 mag/arcsec2, should be regarded formal only, rather than a
manifestation of an extraordinarily dense Dark Matter halo which, as
advocated by these authors, dominates the stellar dynamics of a BCD.
![]() |
Figure 17:
Colors of the stellar LSB host galaxies (filled circles) as
listed in Table 3. For comparison, we show the
color evolution of synthetic stellar populations, calculated with the
GALEV evolutionary synthesis model (Schulz et al. 2002;
Anders et al. 2002, priv. comm.).
An initial mass function with a Salpeter slope and respective lower
and upper stellar mass cutoffs of 0.08 and 100 M![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
The available NIR data allowed not only for the detection and structural analysis of the LSB component for all sample BCDs, but also for the determination of at least one NIR color in the LSB host (Sect. 3.5).
The B-J vs. J-H and
vs. J-H LSB colors (Table 3), whenever available, are displayed in
Fig. 17. For comparison, color predictions from the
GALEV evolutionary synthesis model (Schulz et al. 2002;
Anders, Bicker & Fritze - v. Alvensleben 2002, priv. comm.) are
overlaid in both panels, calculated for stellar populations with
metallicities of
/50 and
/2.5, formed either in an
instantaneous burst or continuously with a constant SFR.
As evident from Fig. 17, NIR colors (J-Hvs.
)
show little evolution for old stellar populations
(
0.1 mag for an age >0.5 Gyr). However, optical-NIR colors,
such as, e.g., B-J (right panel of Fig. 17), change
by up to
1.5 mag for ages >0.5 Gyr. Within the above discussed
errors, such colors will allow to constrain the age of the stellar LSB
host of old iE/nE BCDs with a precision of 1-2 Gyr, once
precise spectroscopic measurements of its metallicity become available
(see the discussion of Gil de Paz et al. 2000b).
Figure 17 shows that the observed colors are generally
compatible with the GALEV predictions within their 1
uncertainties. This is also the case for model predictions from
PEGASE (Fioc & Rocca-Volmerange 1997).
Five BCDs in our sample, Pox 4, Pox 4B, Tol 1400-411, Tol 65 and
Tol 1214-277 (Table 3), show blue (J-H< 0.5,
B-J< 1.5) colors in their LSB component. This may be partly due to an
appreciable contribution of extended nebular emission, as suggested by
previous optical spectrophotometric work. The blue optical-NIR LSB
colors of Tol 65 and Tol 1214-277 may be attributed to a comparatively
young (1 Gyr) photometrically dominant stellar population
(cf. P99, F01). The
optical-NIR colors of the LSB host of the remaining sample BCDs can
be reconciled with an old stellar population of subsolar metallicity.
This result is in line with most published optical-NIR photometry on
stellar hosts of BCDs (Gil de Paz et al. 2000b; Vanzi et al. 2002; Papaderos et al. 2002), which
indicates a comparable range of colors for the LSB population of BCDs.
We can compare the NIR colors of the LSB component with data from the
literature for three sample BCDs, only (Tol 3, Haro 14 and UM 461).
The colors of Tol 3 compare well with those reported by Vanzi et al. (2002)
over the whole radius range of (R* = 0
...50
).
Our photometry for Tol 3 and Haro 14 is also in agreement with
Doublier et al. (2001) for small radii
(for R*
15
and R*
10
,
respectively).
For larger radii, however, the NIR colors by Doublier et al.
are not compatible with our results within the 1
uncertainties.
Outside the star-forming component the color profiles by Doublier et al. approach
values of J-H>2 for Tol 3, and
,
for Haro 14. For UM 461, their color profiles
vary between
(J-H) and
(H-K).
We have analyzed deep Near-Infrared (NIR) J, H, K images of 12
Blue Compact Dwarf (BCD) Galaxies and one luminous Blue Compact
Galaxy. These objects, together with those studied in an accompanying
paper (Cairós et al. 2003), constitute the first part of a sample
of 40 BCDs for which deep NIR images were obtained in the framework of
a large-scale multi-wavelength study. The limiting surface
brightnesses of our data, 23.5 to 25.5 mag/
in J and
22 to 24 mag/
in K, allow for the detection and study of the
NIR structural properties and colors of the underlying stellar
low-surface brightness (LSB) component in all sample galaxies. This
evolved LSB host, underlying the star-forming regions is known to
exist in the majority of BCDs and to contain the bulk of the stellar
mass in these systems.
Consequently, a systematic determination of its structural properties
(e.g., radial stellar surface density distribution, central
surface brightness, exponential scale length) and of the
gravitational potential it forms may prove crucial for the
understanding of the starburst activity, dynamics and evolution
of BCDs. Other than in optical wavelengths, where extended
starburst emission hides the LSB host inside its inner 2-3
exponential scale lengths, NIR studies allow to extend surface
photometry of the underlying old stellar background to a smaller
galactocentric distance, thereby better constrain its overall
intensity distribution.
Our results can be summarized as follows:
Acknowledgements
Research by K.G.N. has been supported by the Deutsche Forschungsgemeinschaft (DFG) Grants FR325/50-1 and FR325/50-2. LMC acknowledges support from the European Community Marie Curie Grant HPMF-CT-2000-00774. We thank U. Fritze - v. Alvensleben, P. Anders, J. Bicker and J. Schulz for kindly providing the GALEV models. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, CALTECH, under contract with the National Aeronautic and Space Administration. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.