Issue |
A&A
Volume 501, Number 1, July I 2009
|
|
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Page(s) | 207 - 220 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200810715 | |
Published online | 05 May 2009 |
The H
galaxy survey![[*]](/icons/foot_motif.png)
VII. The spatial distribution of star formation within disks and bulges
P. A. James1 - C. F. Bretherton1,2 - J. H. Knapen3
1 - Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
2 - Royal Observatory, Greenwich, London, SE10 9LF, UK
3 - Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain
Received 30 July 2008 / Accepted 24 April 2009
Abstract
Aims. We analyse the current build-up of stellar mass within the disks and bulges of nearby galaxies through a comparison of the spatial distributions of forming and old stellar populations.
Methods. H
and R-band imaging are used to determine the distributions of young and old stellar populations in 313 S0a-Im field galaxies out to
40 Mpc. Concentration indices and mean normalised light profiles are calculated as a function of galaxy type and bar classification.
Results. The mean profiles and concentration indices show a strong and smooth dependence on galaxy type. Apart from a central deficit due to bulge/bar light in some galaxy types, mean H
and R-band profiles are very similar. Mean profiles within a given type are remarkably constant even given wide ranges in galaxy luminosity and size. SBc, SBbc, and particularly SBb galaxies have profiles that are markedly different from those of unbarred galaxies. The H
emission from individual SBb galaxies is studied in detail; virtually all show resolved central components and concentrations of star formation at or just outside the bar-end radius.
Conclusions. Galaxy type is an excellent predictor of R-band light profile. In field galaxies, star formation has the same radial distribution as R-band light, i.e. stellar mass is building at approximately constant morphology, with no strong evidence for outer truncation or inside-out disk formation. Bars have a strong impact on the radial distribution of star formation, particularly in SBb galaxies.
Key words: galaxies: spiral - galaxies: structure - galaxies: stellar content - galaxies: nuclei - galaxies: irregular
1 Introduction
Disks of spiral galaxies host the majority of the star formation
activity in the local Universe (Somerville et al. 2001; James et al. 2008; Tinsley & Danly 1980; Hanish et al. 2006, henceforth Paper
IV). However, there are many remaining
questions regarding the formation, stability and growth of stellar
disks, and the interrelation of these with other galaxy components,
particularly bars and central bulge components. Star formation (SF)
is shown by a wide range of indicators to be virtually ubiquitous in
disks, unlike elliptical galaxies where most of the SF was completed
early in their evolution. Typical current SF rates within the disks
of bright, nearby spiral galaxies are 1-2 yr-1(Kennicutt & Kent 1983; James et al. 2004, Paper I), sufficient to accumulate the mass of
a substantial disk if continued over a Hubble time. Thus it is
reasonable to ask whether the spatial distribution of new stars is
consistent with that of the overall stellar mass in disks of the same
type, thus motivating a picture in which disks are constructed largely
through the types of SF we see at the current epoch. Alternatives are
that disks could have grown from the inside outwards
(Trujillo & Pohlen 2005; Muñoz-Mateos et al. 2007), resulting in the youngest stars lying on
average at greater radial distances than the old stellar population;
or indeed outside-in models of galaxy formation have been suggested
(Gallart et al. 2008). It is also possible that spatial distributions of
young and old stars could differ because of radial drifts of stellar
orbits, driven by tidal torques from spiral arms, bars or external
tidal interactions.
The origin of the characteristic exponential profile, found to describe the radial light distribution of many disks, has also been studied through theoretical studies (Freeman & Bland-Hawthorn 2002; Fall & Efstathiou 1980; Elmegreen et al. 2005), most of which start from the observation by Mestel (1963) that the angular momentum distribution of an exponential disk resembles that of a sphere undergoing solid-body rotation.
Observational determinations of the radial distribution of SF in
galaxy disks have been performed in the past, using several approaches
and observational techniques. The approaches include detailed studies
of individual galaxies, e.g., Comte & Duquennoy (1982) who looked at the Sbc
spiral NGC 1566, to statistical studies of typically several tens of
galaxies. An early study of the latter type was carried out by
Hodge & Kennicutt (1983), who considered the distributions of H II regions
revealed by narrow-band H
imaging, finding them to have
characteristic ring or ``doughnut'' shaped distributions in early-type
spirals, with more extended distributions being found in late-type
spirals, and ``oscillating'' (non-monotonic) distributions occurring in
many barred galaxies. Ryder & Dopita (1994) studied the relative scale
lengths of H
,
V- and I-band emission from 34 S0-Sm
galaxies, finding the line emission to have larger scale lengths than
those of the continuum emission. García-Barreto et al. (1996) investigated
H
imaging of 52 barred spiral galaxies, finding nuclear rings
in 10, and emission indicating SF along the bar in 18.
Dale et al. (2001) analysed H
extents of galaxies normalised by
their I-band sizes, finding no strong type dependences; similar
results for H
to R-band scalelength ratios were found by
Koopmann & Kenney (2004) and Koopmann et al. (2006).
Hattori et al. (2004) studied the effects of galaxy-galaxy interactions on the
distributions of H
emission within disks, finding that extended
starbursts are common in such galaxies.
Finally, Bendo et al. (2007) looked in detail at the SF distribution in
galaxies from the SINGS sample, using 24
m emission as the primary
SF tracer. They found this emission to be typically compact and symmetric
for early type galaxies, but more extended and asymmetric for late type
galaxies.
The aim of this paper is to address these questions through an
analysis of the spatial distribution of SF as traced by the
H
emission line in a large sample of local disk and irregular
galaxies, and to compare it with the distribution of older stars.
H
provides essentially a snapshot view of SF activity, since it is
powered by massive stars with main sequence lifetimes of
107 years. This can cause problems with the interpretation of the
H
properties of individual galaxies, since the emission is driven
by a small number of H II regions with short lifetimes, and so the
stochastic uncertainties are substantial. Thus the approach adopted
here is to look at the mean properties of at least several galaxies of
the same type, giving a statistical basis for studies of the
distribution of SF, the growth of disks, and the effects of bars on
the SF process. The galaxies used are a field sample, and hence the
effects of environment should be small, though some of the galaxies do
have fairly close companions. These will be studied in a subsequent
paper (Knapen & James 2009, Paper VIII).
All data are taken from the H
Galaxy Survey (H
GS), a survey of
327 nearby galaxies (plus a further 7 serendipitously observed
objects) which have been imaged in both the H
line and the R-band
continuum. The narrow-band filters used encompassed the H
and
neighbouring [N II] lines, which should be borne in mind when
interpreting the narrow-band flux distributions in the present paper.
For convenience, this combined emission is referred to as
H
throughout. The H
GS sample contains all Hubble types from
S0/a to Im and galaxies are selected to have heliocentric
recession velocities less than 3000 km s-1. All galaxies were
observed with the 1.0 m Jacobus Kapteyn Telescope (JKT), part of
the Isaac Newton Group of Telescopes (ING) situated on La Palma in the
Canary Islands. The selection and the observation of the sample are
discussed in Paper I. The overall aim of the survey is to quantify as
fully as possible the star formation properties in field galaxies at
the current epoch. Earlier H
GS papers have looked at total SF rates
and H
equivalent widths in galaxies (Paper I), and the
contributions of galaxies of different types to the integrated SF rate
(SFR) per unit volume of the local Universe (Paper IV). Whereas
previous papers derived from the H
GS have mainly focussed on
integrated SF properties for each of the galaxies studied, the present
paper will focus on the spatial resolution of SF provided by the
H
imaging technique. This will be done first through an analysis
of three concentration indices applied to both the R-band and
H
light distributions, and then through more detailed study of mean
normalised radial light distributions for galaxies of each
morphological type, and for barred and unbarred galaxies.
Some results from the earlier papers that are relevant for the
present study will first be summarised. Paper I contained a brief
analysis of the effect of bars on total SFRs of galaxies, and on the
equivalent width of H
emission. Bars give modest, barely
significant increases in both quantities overall; however, barred
galaxies of types Sab-Sc inclusive were found to have larger SFRs by
factors 1.5-2.0 compared with unbarred galaxies of the same types.
The SFR per galaxy is highest in intermediate disk types with
classifications of Sbc and Sc. In Paper IV it was found that
these same types also dominate the SFR density, i.e., SFR per unit
volume of the local Universe, summed over all Hubble types. Disk
regions, defined as those lying more than 1 kpc from the centres of
galaxies, were found to contribute more than 80% of the total SF
currently occurring in the local Universe.
![]() |
Figure 1:
The concentration index C30 plotted against
Hubble type T. The plot on the left shows the mean value of the
R-band C30 index for each type, with the error bars representing the
standard error on the mean for all
galaxies of that type. The plot in the centre shows the same mean
concentration indices for H |
Open with DEXTER |
Sample H
and R-band images of several galaxies from the H
GS
database are presented in Paper I, and a large number of H
images
of those galaxies that have hosted supernovae are presented in the
online version of another paper in this series
(James & Anderson 2006, Paper III). These
provide a good indication of the overall quality of the data used in
the present analysis, and in the interests of brevity no images are
shown in the present paper.
The structure of this paper is as follows. Section 2 contains an
investigation of three concentration indices, which are applied to
both H
and R-band light distributions. The strength of
correlation between indices and Hubble type is studied, leading to
preliminary conclusions on the relative distributions of young and old
stellar populations as a function of galaxy type. The effect of strong
bars on the radial distribution of SF as traced by these indices is
also analysed. Section 3 looks at mean radial profiles as a more
detailed tracer of stellar distributions. H
and R-band mean
profiles, binned by type, are presented for the full sequence of
spiral types, and compared for barred and unbarred types, leading to
the identification of an effect on profiles for bars in SBb types that
is particularly marked in H
profiles but also clear in the R-band
light distributions. The effect of continuum-subtraction errors
on profiles is also studied in this section. Section 4 contains a
detailed investigation of the central and bar-end H
emission in SBb
galaxies. Section 5 contains a discussion of some of the main
results, and the conclusions are presented in Sect. 6.
2 Concentration indices
2.1 The concentration indices studied here
Concentration indices provide a simple measure of the observed radial distribution of the luminosity of a galaxy in a specified bandpass. They are of use since they contain much of the information contained in a galaxy classification (as will be demonstrated below) but in a quantified and less subjective form. The three indices investigated here are all measures of the ``cuspiness'' of the light distribution, and are sensitive to the difference between, for example, an r1/4 law profile characteristic of an elliptical galaxy or a luminous classical bulge, where a large fraction of the light is concentrated in a central spike, and a more extended distribution such as an exponential, as would be expected for a galaxy disk. All are scale-independent, and can be applied without distance information.
The first index is the C30 index of Abraham et al. (1994) and Koopmann & Kenney (1998), which is the ratio of the flux within 0.3 times the R = 24.0 mag per square arcsec isophotal radius (henceforth r24) and the total flux within that same radius.
Secondly, the C31 index (de Vaucouleurs 1977) is defined as the ratio of the radii of the apertures containing 75% and 25% of the total light, although for convenience here we use the log to the base 10 of this quantity. Other variants make use of different fractions of light, with 80% and 20% also being widely used. One drawback of C31 is that it requires an estimate of the total luminosity, which is generally a poorly defined quantity.
Finally, we study
the Petrosian index
(Shimasaku et al. 2001) which is based on the Petrosian
radius (Petrosian 1976) of the galaxy under study. This is the radius
at which the local surface brightness (SB) is fainter by a given
factor
than the average SB within that radius. This is a
useful quantity since this radius is independent of galaxy distance,
K-corrections and extinction (if uniform). Here we define the
Petrosian radius for an index
0.2; the concentration index is
then the ratio of the radii containing 50% and 90% of the flux
within the Petrosian radius.
In this section we investigate the correlation between these different
indices and galaxy Hubble type for the HGS sample; the sample size
is sufficient for us to separate galaxies by Hubble T-type (de Vaucouleurs 1959),
with the
latter running from T=0 (S0a) to T= 10 (Im). The data used are
multi-aperture photometry from the CCD R-band and narrow-band
continuum-subtracted H
images from the H
GS survey. For the
spiral types S0a-Sm (T-types 0-9), elliptical apertures are
used, with constant ellipticity and position angle at all values of
the aperture semi-major axis; these parameters are defined by galaxy
isophotal shapes in the outer regions of their disks. For Im
Magellanic irregulars of type T=10 (and face-on spirals), circular
apertures are used.
2.2 Concentration indices for R-band and H
light as a function of galaxy type
2.2.1 The C30 index vs. type
![]() |
Figure 2:
The difference in mean concentration index between
unbarred/weakly barred galaxies and strongly barred galaxies,
plotted against Hubble type T. For
points lying above the line, the unbarred/weakly barred galaxies
have more centrally concentrated emission than do their strongly
barred counterparts. The plot on the left shows the mean
difference in this index for the H |
Open with DEXTER |
Figure 1 shows the mean values of the C30
concentration index for R-band and H
light (left and central
plots), with the right hand plot showing the difference between the
R-band and H
mean indices. The R-band C30 index plot shows
small error bars, and hence a small scatter in the index, for each
Hubble T-type, both in an absolute sense and relative to the
variations between T-types. This correlation of C30 with Hubble
type was investigated using two methods. Firstly, a non-parametric
Spearman rank analysis yields a
value of 0.98, well above the
99% confidence value which requires a
of 0.79 or greater.
At the referee's suggestion, here and elsewhere we calculated an
error-weighted fit assuming a linear model for the relation between these
parameters. The significance of the deviations from this fit is 96%
(4% probability of these residuals arising by chance).
A similar correlation was also found by Koopmann & Kenney (2004,1998)
for a sample of 29 isolated galaxies of types S0-Sc (they found
interestingly different results for Virgo cluster galaxies but that
sample is less relevant here). Koopmann & Kenney (2004) report C30 values
decreasing smoothly from 0.61-0.72 for S0 galaxies (slightly earlier
than any galaxies in the present sample) to 0.24-0.37 for Sc types.
Thus the range they find is very similar to that of the present study,
although their Sc indices correspond more closely to those of our Sd-Im types.
The index applied to the H
light
distribution shows a substantially larger scatter, but an overall trend
that is still significant according the Spearman test (
0.94).
The significance of deviations from the best-fit linear trend is
again 96%.
For most types, the R-band
light is more centrally concentrated in the mean than the H
light.
This is as expected given that
most star formation takes place in the disks of galaxies, whereas
bulges contain predominantly old stars.
Types Sa and Sab are exceptions to this trend, showing marginally the
opposite result. In this index, the latest types (Im; T= 10) show
no significant difference in the mean R-band and H
indices.
2.2.2 The C
index vs. type
Secondly we investigated the mean values of C31 binned by galaxy
type, for both R-band and H
emission. An exponential profile has
a concentration index of 0.447, whereas an r1/4 law profile has
one of 0.845. The mean concentration index variation as a function of
galaxy type shows a very similar pattern to the C30 index so we omit
the equivalent figure. The bulge-dominated S0/a and Sa galaxies have
the highest values of R-band index,
,
intermediate
between the expectations for pure disk and pure r1/4 bulge, as
would be expected. The later type, disk-dominated, spiral galaxies
and irregulars have indices
,
similar to those
expected for pure exponential profiles, in accordance with the
findings of Kent (1985).
The correlation between R-band
C31 index and T-type is significant, with a Spearman
of 0.99.
The significance of deviations from the best-fit linear trend is 99%.
The mean H
C31 concentration index also shows a clear
relationship with galaxy classification, with the late-type spirals
and irregulars possessing values lower than the early types. Again,
there is a larger scatter in H
indices than for the R-band. For
the later spiral types, Sc-Im, the H
concentration index is
lower than that of a pure exponential.
The overall relation
between H
C31 index and T-type yields a Spearman
of 0.91
indicating high significance.
The significance of deviations from the best-fit linear trend is 71%.
We also investigated the difference
between R-band and H
C31 indices. In most cases this
difference is positive, implying that the continuum light is more
centrally concentrated than the H
.
However, for this index, the higher concentration
of old stars compared with star formation occurs even for the latest
Hubble types, including the Magellanic irregulars, which are not
thought to contain significant bulge components.
Table 1: Number of galaxies of each Hubble T-type and bar classification contributing to the mean profiles presented in this paper.
2.2.3 The Petrosian index vs. type
The Petrosian index is an inverse concentration index, so a more
centrally-concentrated galaxy will have a lower value of
.
An
exponential surface brightness profile has an index of 0.501 and
an r1/4 law profile results in a
value of 0.371.
Again we find the bulge-dominated S0/a and Sa
galaxies to have light profiles that are close to the value expected for
a pure r1/4 law,
whereas the irregulars and disk-dominated spirals have concentrations
that are closer to that predicted for a pure exponential profile.
The relation between R-band
index and T-type
is found to give a Spearman
of 0.98, again indicating strong
correlation.
The significance of deviations from the best-fit linear trend is 99.7%
A similar analysis is presented Fig. 10 in Shimasaku et al. (2001) which shows
the Petrosian concentration indices for 426 SDSS galaxies against
their T-type classification. The values for the HGS galaxies appear to
be systematically higher than those for the SDSS galaxies.
The majority of SDSS Sa-Im galaxies have indices between 0.35 and
0.50, whereas the equivalent range for H
GS galaxies is between 0.44
and 0.57. The reasons for this offset are not clear, but it should be
noted that the H
GS sample contains a large proportion of low luminosity
and low surface brightness galaxies, particularly amongst the late types,
which may be less fully represented in other samples.
The H
Petrosian concentration index again has a less clean trend as
a function of galaxy type than the R-band index, but the
Spearman rank test still indicates a strong correlation,
0.90.
The significance of deviations from the best-fit linear trend is 92%.
For most types, the continuum emission
appears more centrally concentrated than the H
emission. Sab and
Sb galaxies are the exception and appear to have statistically similar
light distributions in R-band and H
emission.
Overall we conclude that the C30 index seems marginally the best proxy for morphological type of the three investigated here.
2.2.4 The C30 index as an indicator of bar presence
We also carried out an analysis of the effect of bars on the mean
concentration indices. Of the three indices discussed above, the C30
index was preferred for this analysis as a result of the small scatter
about mean values found above for this index. Figure
2 shows the difference in the mean value of this
index between unbarred or weakly barred and strongly barred galaxies
(i.e., A and AB vs. B classifications). The left-hand plot shows the
effect of bars on the H-derived index, whereas the right-hand plot
is for R-band emission. The error bars show the standard error on
the mean value of the difference for each type. For points lying
above the line, the unbarred/weakly barred galaxies have more
centrally concentrated emission than do their strongly barred
counterparts. For the R-band plot, this appears to be the case for
most spiral types: all the points for T= 0-7 have positive values,
indicating that strong bars are associated with less centrally
concentrated R-band luminosity. For the later types (T= 8-10)
any effect of bars seems to be in the opposite sense, with the
R-band light being somewhat more centrally concentrated than for
those galaxies with no or weak bars. The relation between bar
effect and galaxy T-type was investigated using the two
statistical tests introduced above. The Spearman test indicates a
significant correlation, with the
value being 0.82.
The significance of deviations from the best-fit linear trend is 71%.
Similar trends are seen in the left-hand plot of
Fig. 2 for the distributions of H
light. For all
T-types, the scatter is larger, but again the earlier type spirals
show less central concentration in the barred galaxies, with the
reverse being true for later types. In this case the transition takes
place between types 5 and 6 (Sc and Scd), although given the size of
the error bars the differences for any individual type are not
significant. The Spearman test indicates that this correlation
is significant (
).
The significance of deviations from the best-fit linear trend is 38%.
The main result from this initial analysis of barred galaxies is the lower central concentration of both old and forming stars in earlier-type barred spiral galaxies than non-barred. This effect will be studied in more detail in Sect. 4.
3 Mean light profiles in R and H
light
![]() |
Figure 3:
Normalised mean profiles in H |
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3.1 Calculation of mean normalised light profiles
The concentration indices considered in the previous section convey only a small fraction of the information on the spatial distribution of luminosity that is contained in our images. In this section, we will present an analysis that attempts to extract more of that information, whilst still enabling meaningful averages to be taken, thus minimising the wide variations in properties that plague studies of small numbers of galaxies. This analysis makes use of normalised light profiles, which we calculate for each galaxy based on the elliptical aperture photometry that was used in the previous section to derive concentration indices. However, we choose not to present these in the form of surface brightness profiles, in units of magnitudes per square arcsec, but rather use the fluxes in the elliptical annuli, uncorrected for the increasing area of the annulus as the apertures grow. Thus the area under such a profile within a given range in radius is directly proportional to the amount of light contributed to the total luminosity of the galaxy. The shape of the resulting profile is much more intuitively related to concentration indices than is a surface brightness profile, because of this link between flux and area under the profile. For example, the effective radius can be simply estimated from such a plot as the radius which evenly divides the area under the profile.
It is necessary to normalise such profiles before combining them into,
for example, a mean profile for all galaxies of a given type. A
simple average without normalisation will be dominated by the
brightest galaxies; and if a radial scale in kpc is adopted, then all
galaxies will contribute to the centre of the mean profile, but only
the largest galaxies to the outer regions, giving a distorted result
that is hard to interpret. Thus we normalise the individual profiles
obtained from each galaxy in two ways. Firstly, the radial scale is
expressed in units of the r24 isophotal
semi-major axis of the galaxy. Secondly, the area under each profile
is normalised to unity, giving each galaxy, bright or faint, equal
weighting in the mean profile. This process was applied first to the
R-band image of each galaxy, and then to the H
image using
exactly the same positions, shapes and size of apertures, and scaling
again by the R-band isophotal size. As a result of the normalisation,
the profiles have no calibration in flux or magnitude units on the
vertical axis; the profiles are simple shape functions, indicating
the fraction of light residing in a given radial range.
In the remainder of this section and in the next, we look at the mean
profiles produced by this process, and examine the dependence of profile
shape on galaxy type, and on the presence or absence of bars. The
mean profiles for the individual T-types contain between 8 and 67
profiles, as listed in Table 1, which also subdivides the
numbers for each type according to bar classification. Some galaxies from
the HGS sample were omitted from this analysis due to contamination
by foreground stars or problems with varying background levels across
images. Hence the total number of galaxies listed in Table 1
is 313, out of the full H
GS sample of 327.
3.2 Mean profiles as a function of Hubble type
The mean profiles for all galaxies as a function of Hubble T-type
are shown in Fig. 3, with the H
profiles on the
left and the R-band profiles on the right. The number of galaxies
contributing to each profile is indicated; it is important to note
that all 313 profiles of sufficient cosmetic quality have
been included, so there has been no exclusion of outliers with unusual
profile shapes. Error bars indicate the standard error of the
individual scaled profiles about the mean at each radial point. The
errors on the H
profiles are significantly larger than those on the
R-band profiles, originating from the clumpier nature of the
H
emission.
3.2.1 R-band mean profiles
The main point to note from the R-band profiles in
Fig. 3 is the degree of correlation between mean
profile shape and Hubble type, which is quite gratifying given the
subjective nature of galaxy classification. The mean profiles of the
earliest types (T-types 0 and 1) are very centrally concentrated,
with the peak of the profiles, and hence the largest contribution to
the total galaxy luminosity, coming from 0.1 r24. The one
anomaly in an otherwise smooth sequence of R-band profiles is for
the Sb galaxies (T= 3); the additional light in this mean profile
close to the centre and at 0.4 r24 will be discussed in
Sect. 4. For later types, the emission systematically
moves outwards and broadens, with the peak sitting at
0.4
r24 for the Sm and Im galaxies (T-types 9 and 10). This smooth
progression in profile properties is shown in Table 2,
which lists the peak and effective radii for the R-band and
H
mean profiles for each T-type, in units of r24 (the mean
value of r24 for each type is also given in kpc in the final
column, to enable approximate conversion of these values to physical
units, although it should be noted that there is a large range of
galaxy sizes present at each type). The same data are plotted in
Fig. 4. Sizes in kpc are calculated using an
effective Hubble constant of 75 km s-1 Mpc-1, corrected
for Virgo infall as explained in Paper I.
All four parameters plotted in Fig. 4 are strongly
correlated with T-type, with Spearman
values between 0.83 and
0.95.
Deviations from best-fit linear models have significances
of 64-94%.
Table 2:
Peak and effective radii of the H
and R-band mean profiles,
in units of the R24 isophotal radius; and the mean value of this
radius, in kpc, all listed as a function of T-type.
![]() |
Figure 4:
Peak and effective radii of the H |
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3.2.2 H
and (H
- R) difference profiles
The mean H
profiles are predictably less smooth than those for the
R-band emission, but overall show a very similar trend of decreasing
central concentration from early to late types. In order to emphasise
any differences between the H
and R-band mean profiles, we
constructed difference profiles, in the sense H
- R, from those
shown in Fig. 3; the results are shown in
Fig. 5. For the late-type spirals and Magellanic
irregulars, these plots have the expected form and are simple to
interpret: late-type spirals (T between 4 and 9 inclusive) show
statistically similar distributions of R-band and H
emission
outside
0.5 r24, but within this radius the relative
strength H
emission dips, presumably due to the influence of
predominantly old stellar populations associated with
central bulge or bar components. This ``bulge dip'' in the
H
emission appears to be the explanation for the lower
H
concentration indices found for T-types 3-9 in the right hand
plot of Fig. 1.
For the Magellanic irregulars (T= 10), the star formation traces the
R-band light at all radii, and there is no trace of a central older
population (previously described in Paper V); this is clear evidence
that these are bulge-free galaxies. It is important to note here the
need to average over many tens of galaxies for this agreement to
emerge. Individual H
profiles of individual Im galaxies are very
broken and spikey, since there are typically only a few H II regions
per galaxy, so a statistical approach is needed to reach this
conclusion. It is somewhat surprising that the central suppression of
SF is clearly seen in the latest spiral types, even the T=9 Sm
galaxies; these might also have been thought to be ``bulge-free'' prior
to this analysis.
![]() |
Figure 5:
Difference profiles, H |
Open with DEXTER |
The most surprising finding from Fig. 5 is that the
earliest types, T= 0, 1 and 2, which should contain the most
dominant bulges, do not show the expected central depression in their
H
- R profiles. For the T= 0 (S0a) galaxies the difference
profile is consistent with being flat, within the rather large error
bars, whereas the Sa and Sab profiles (T= 1 and 2) show emission
line excesses, relative to the R-band light, out to radii of
0.3 r24, which corresponds to
3 kpc for the average
size of galaxy contributing to these profiles. The nature of such
extended emission is not clear. Active Galactic Nuclei (AGN) and
Low-Ionization Nuclear Emission-line Regions (LINERS) are common in
galaxies of these types (Ho et al. 1997), but the line emission from such
nuclei should be unresolved in our profiles. Hameed & Devereux (1999) report
the finding of what they termed Extended Nuclear Emission-line Regions
(ENERs) in 7 out of 27 Sa and Sab galaxies for which they had
narrow-band H
+ [N II] imaging. They speculated that this
emission arises from gas excited by post-Asymptotic Giant Branch
stars, which would lead to the expectation that such emission should
be distributed like the bulge stellar luminosity. James et al. (2005, Paper
II) confirmed that central emission-line components with the
same smooth morphology reported by Hameed & Devereux (1999) are present in H
GS galaxies, and demonstrated that in at least one case the emission
is dominated by the [N II] line and not H
.
Such emission is
qualitatively very different in appearance from the H
emission from
SF regions; the former has a smooth, diffuse and centrally symmetric
structure, whereas SF regions are characteristically very clumpy and
irregular. In the present sample, extended central emission regions
which may be ENERs are seen in 9 out of 62 galaxies with T= 0 to 3,
and are very rarely detected in later-type galaxies, possibly due to
the dominance of the emission by that from SF. Circumnuclear rings of
SF also predominantly occur in galaxies of types 3-4. They occur in
up to 20% of local spiral galaxies (Knapen 2005), and at higher
frequencies in types 3-4. With radii varying from a few hundred
parcsec to some 2 kpc, they may also make a substantial contribution
to a central peak in H
emission in individual cases.
Outside the bulge regions, the mean H
and R-band profiles of each
type are essentially identical in their overall morphologies. Thus
the disk SF appears to have a very similar radial distribution to the
older stellar population traced by the R-band light. Minor
differences are apparent: the H
profiles for T> 3 peak at slightly larger
radii than the R-band profiles, and the effective radii of the H
profiles
also tend to be larger (see Table 2). Both can
be accounted for by the central suppression of H
emission in bulge
regions. Outside these regions, in the disk-dominated parts of the
profiles, the H
and R-band light distributions are identical
within the errors. Thus there is no evidence of, for example, the
radial truncation of H
emission found by Koopmann & Kenney (2004) for a
sample of 55 Virgo cluster spirals. This is consistent with their
interpretation that the truncation is a consequence of the cluster
environment (they found no truncation for a comparison sample of 29
isolated spiral galaxies), since our sample is predominantly composed
of field galaxies. There is also no evidence from these profiles to
support inside-out theories of disk formation; the old stellar
population could have been produced by historical SF distributed like
that occurring at the present epoch. The possibility of large-scale
radial migration of disk material driven by, for example, bar torques
obviously complicates this conclusion.
3.2.3 The effect of continuum subtraction errors on H
profile shape
One of the most important, and problematic, stages in the reduction of
narrow-band imaging is the removal of the continuum light which passes
through the narrow-band filter in addition to the desired line
emission. For HGS this was done using additional imaging through
either a broad R or an intermediate-width continuum filter. The
procedures and associated (significant) errors on derived total fluxes
are explained in Paper I. The possibility of errors leading to
under- or over-subtraction of the continuum light is particularly
important for the present paper as, for example, systematic
under-subtraction could easily lead to a spurious apparent agreement
between the shapes of R-band and ``H
'' profiles due to possible
red-light contamination in the latter. A test was
performed to determine the impact of continuum subtraction errors on
normalised H
profile shapes, for 10 galaxies from H
GS covering the
full range of types. For each one, the scaling factor applied to the
continuum image was varied by
times the statistical error on this
parameter, such that the residuals of
unsaturated stellar images on the subtracted frame would be all negative or
all positive in the two resulting frames.
Thus we took conservative cases of clearly over- and under-subtracted
images for this analysis.
H
profiles were then produced
for each of these pairs of frames, using the methods described above.
![]() |
Figure 6: Profiles made with the continuum light deliberately over- and under-subtracted (dot-dashed and dashed curves respectively), for five representative sample galaxies: UGC 4097 (Sa), UGC 4273 (SBb), UGC 6644 (Sc), UGC 6778 (SABc) and UGC 8490 (Sm). |
Open with DEXTER |
The results of this analysis are quite reassuring. Even though
continuum subtraction errors can alter total H
fluxes by
30%, the effects on the shapes of profiles, after renormalising
the profile area to unity, is much smaller. Figure 6
shows, for a selection of the 10 galaxies studied, that the strongest
effects are unsurprisingly found in the central regions of early-type galaxies,
since these are dominated by high surface brightness bulges. However, even
in these cases the overall shapes of the derived H
profiles are not
greatly affected. The H
effective radii for UGC 4097 and UGC 4273,
which show the greatest variation in profile shape in
Fig. 6, both change by
% as a result of this
degree of continuum over- and under-subtraction. For galaxies of type
Sc or later, the effects of continuum subtraction errors on profile shape are
negligible.
3.2.4 Dependence of profiles on galaxy distance, size and luminosity
![]() |
Figure 7: Mean R-band light profiles for T= 5 galaxies ( left) and T= 9 galaxies ( right), with the 6 profiles in each frame showing the bright, faint, large, small, far and near halves of the total sample from top to bottom. |
Open with DEXTER |
The strong dependence of R-band profile shape on T type, and the generally small error bars on the R-band profile points in Fig. 3, indicate a high degree of uniformity of light profiles which is somewhat surprising given the range of properties of the galaxies contributing to each profile. The 36 galaxies contributing to the T= 5 profiles lie at distances ranging from 2.1 to 34 Mpc, and have R-band absolute mags from -16.1 to -21.6, corresponding to a factor of 160 in luminosity; for the 42 T= 9 galaxies, the distance range is similar, and the luminosity range still larger at a factor of 360 (MR from -12.9 to -19.3). We have thus checked for any dependence of the mean profile shape on distance, galaxy radius in kpc, and luminosity, using these two types as test cases due to the large numbers of profiles available and the large range of properties included in each type. The results are shown in Fig. 7. Each panel shows the mean R-band profiles for one half of the galaxies of the appropriate type, with the split being done by distance, galaxy radius and R-band luminosity. T= 5 galaxy profiles are on the left, those for T= 9 on the right. For T= 5, the only effects seen are the appearance of a subtle ``shoulder'' at 0.5 r24 in the bright, large and distant halves of the sample, but the overall shapes of the profiles are all very similar. For T= 9, the bright and large halves of the sample have mean profiles that are somewhat more centrally concentrated than the faint half. The bright half matches the overall T= 8 profile in peak position (0.34 r24), and has an effective radius (0.64 r24) between those for the T= 8 and T= 9 profiles. Thus the luminosity dependence appears slight, and overall we conclude that profile shape is largely determined by galaxy classification.
4 The effects of bars on distributions of star formation and continuum light
4.1 Mean light profiles for barred and unbarred galaxies
![]() |
Figure 8:
Mean H |
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![]() |
Figure 9: As Fig. 8 but showing the R-band profiles, again subdivided by bar classification. |
Open with DEXTER |
The mean R-band profile for Sb galaxies (T= 3) in Fig. 3 shows a distinctive ``shoulder'' on the central
emission peak, extending to a radius of 0.5 r24. The same
effect is seen, albeit less strongly, in the mean Sbc profile. The
mean H
profiles of these two types are also distinctive, with a
narrow central peak, a minimum in the profile (not seen for any
other types), and an outer maximum which is at an anomalously large
radius given the relatively early types (see Table 2).
This is particularly marked for the Sb mean profile. The size
corresponding to the ``shoulder'' radius for the Sb profile is 4.8 kpc,
which immediately suggests that the excess light may be due to bars.
Indeed, 13 of the 23 galaxies contributing to this mean profile are
classified SBb. To investigate this possibility, in
Figs. 8 and 9 we have replotted the
H
and R-band mean light profiles of Fig. 3
subdivided according to bar type. In each figure, the left-hand plots
show mean profiles for galaxies classified as showing no evidence for
bars in their optical morphologies (St/SAt, or Im); the central
plots show those for intermediate or possible bar types (SABt or
IABm); and the right-hand plots the profiles for galaxies with clear
optical bars (SBt or IBm). Bar classifications are taken from
de Vaucouleurs et al. (1991) who used optical images; it should be noted that near-IR
imaging can reveal weak or small bars not seen in the optical.
In order to maintain reasonable numbers in
each mean profile, early (T= 0-2) and late (T= 6-9) spiral types
have been combined. Types 3-5 are plotted separately as these are the
most important for the following discussion.
Figures 8 and 9 confirm that the
distinctive profiles seen for the T= 3 galaxies are indeed due to
the barred SBb galaxies. The mean H
profile for the SBb galaxies
shows a very strong central peak of emission, and a clear outer peak
at 0.5 r24. Both are absent in the mean profile of the Sb
(unbarred) galaxies, and indeed from the mean profiles of the unbarred
galaxies of any T type. The profile for the T= 3 SABb galaxies is
noisy, given that only 5 galaxies contribute, but appears intermediate
between those of the Sb and SBb types, with some evidence of a central
peak. The R-band mean profiles for Sb, SABb and SBb types show the
same pattern, with the characteristic bar features being somewhat less
strong in the older stellar population. The SBb R-band profile
still has a central peak, and an outer peak at 0.5 r24, but both
represent a much smaller fraction of the overall flux than is the case
for the mean H
profile. Bars thus have a substantial effect on the
R-band light distribution for T= 3 galaxies, with the outer peak
pushing a large fraction of the flux further out in these galaxies, an
effect which explains the lower concentration indices for barred
galaxies illustrated in Fig. 2. Indeed, the effect
on the concentration indices would presumably be even more marked were
it not for the central peaks in the mean barred profiles, which must
act to increase the concentration indices.
These characteristic bar profiles occur for T= 3 galaxies where they
are most apparent, but also for T= 4 galaxies, and possibly for
those of T= 5. (In comparing the T= 3 and 4 barred mean profiles,
it should be noted that there are 13 SBb galaxies contributing to the
mean profiles against only 4 of type SBbc, so inevitably the mean
profile for the latter type is more noisy.) The mean H
profile
for the SBbc (T= 4) galaxies is clearly different from that for the
SBc (T= 5) galaxies, with the former again having a central peak and
an outer peak at
0.5r24, like those in the SBb mean
profile. The SBbc R-band profile shows only a weak central peak,
and the outer profile shows a ``shoulder'' of flux pushed to larger
radius, but no outer peak as such. For T= 5, the only effect is the
broadening of the mean profiles, with flux again pushed to larger
radii. The H
profile is noisy at all radii, but the R-band
profile shows evidence for this additional flux causing a ``shoulder''
in the profile. No central or outer peaks are seen in the T= 5 mean
profiles, or indeed in any of the profiles for types later than this.
Thus we have identified a clear effect of bars on the pattern of SF as
a function of radius within galaxy disks, which results in strongly
enhanced H
emission, and moderately enhanced R-band emission, in
both the central regions and at 0.5 r24. This effect seems to
apply most strongly to those galaxies classified as SBb or SBbc (T=3, 4), where the overall distributions of SF are profoundly different
from their unbarred counterparts. It should be recalled that the mean
profiles we show here are constructed such that areas under the
profiles are directly proportional to fractions of total flux. Thus
inspection of the SBb mean H
profile shows that a significant
fraction of the total H
+ [N II] emission from these galaxies is
associated with the outer peak feature.
The greater strength of the bar-induced features in the H
mean
profiles than in the R-band profiles is important, since this
confirms that the bars are inducing SF that would not otherwise be
happening, and are not merely redistributing pre-existing stellar
populations. If the latter were the case, the amplitude of the effect
would be the same in R-band light and H
(see Seigar & James 2002, for a
similar argument applied to the triggering of SF by spiral arms).
Having identified this characteristic pattern of SF in SBb galaxies,
we next investigate the nature of both nuclear and outer
emission peaks, by looking in detail at the continuum-subtracted H
images for all galaxies of this type in the H
GS sample.
4.2 Individual SBb galaxies: nuclear emission peaks
Table 3: Properties of the central emission peaks.
To study in more detail the origin of the central peak in H
in SBb
(T= 3) galaxies and, in the next subsection, that of the peak near
0.5 r24, we consider here the properties of the 14 individual
galaxies that make up this subsample.
The properties of the central emission-line peaks seen in these 14
galaxies are listed in Table 3
(UGC 6123, which was omitted from the mean profiles because of a
bright superposed star, is reinstated in this table; note that this galaxy
does show both nuclear and bar-end emission). The major
conclusion to draw from Table 3 is the near-ubiquity
of nuclear peaks, which are found in 13 of the 14 SBb galaxies (denoted
by ``Y'' in Col. 5).
Table 3 also contains information on the sizes
of the central peaks.
That for UGC 3685 is unusual in that it is unresolved in our
H
image (``U'' in the Col. 6), i.e. the FWHM of the peak is
consistent with that of stellar images in the same image before
continuum subtraction. This favours an AGN interpretation for this
emission, as a typical SF complex would be resolved at the distance of
this galaxy. All the other central profiles are marginally resolved (``R?'') or
resolved (``R''; ``R+'' implies the source is several times
larger than the seeing disk). The approximate diameter of the
emission line region is given in Col. 7. For the resolved regions
this is typically a few hundred pc, which would imply the presence of
SF complexes or nuclear rings of SF. However, this does not exclude
the possibility of AGN emission in many or all of these galaxies, and
indeed four of those with extended emission are known to have
LINER-type nuclei. In two of these (UGC 7523 = NGC 4394 and
UGC 7753 = M 91) the extended emission has the smooth morphology
characteristic of ENERs.
Two of the galaxies in Table 3 will be discussed in more detail in Paper VIII; these are the luminous interacting galaxy NGC 7714, and NGC 3769.
4.3 Individual SBb galaxies: outer emission peaks
Table 4: Properties of the outer emission peaks.
The properties of the regions contributing to the line emission at a
radius of 0.5 r24 are summarised in Table
4. Here the fourth column indicates whether the
individual H
profile for the galaxy concerned exhibits such an
outer peak, Cols. 5 and 6 give the radius of the peak in units of the
r24 radius and kpc respectively; and the final column contains
descriptors of the morphology of the emission-line regions leading to
this peak, obtained through inspection of the continuum-subtracted
H
images. ``R'' denotes a ring morphology, ``RB'' a broken or partial
ring, ``BE1'' emission predominantly from one end of the bar, and ``BE2''
emission from both bar ends.
In this case we find that 11 of the 13 galaxies contributing to the mean SBb profile have clear outer peaks in their mean light profiles. The contaminated image of UGC 6123 shows emission from the one end of the bar that is clearly visible, but the foreground star makes it impossible to decide between ``BE1'' and ``BE2'' designations for this galaxy.
5 Discussion
5.1 Concentration indices
Three different concentration indices have been investigated in
Sect. 2. All three methods show a strong
correlation with Hubble type, with the early-type, bulge-dominated
galaxies having concentration indices close to those expected for an
r1/4 profile and the late types approximating to those of
exponential profiles. Investigation of the relative concentrations
finds that most galaxies are more centrally concentrated in the
continuum light than in the H.
Strong bars tend to decrease the
central concentration of R-band light, and to a smaller extent the
H
light, over most disk types. For the latest type barred galaxies
(SBdm-IBm) the reverse trend is seen.
5.2 Radial profiles
From our radial flux profiles, we identify two features that are seen in barred galaxies of morphological types T= 3 and 4 but not in un-barred galaxies of those types, which are a central peak of emission and a second peak at 0.5 r24. As was was discussed in Sect. 4.2, the nuclear peak is most probably due to nuclear emission not related to SF, ENERs, and/or circumnuclear rings. The second peak occurs just outside the end of the bar, and may be related to the presence of an inner (pseudo-ring) as known to exist in many galaxies.
From the mean H
profile, we can estimate the overall
importance of the SF related to these features. We find that
50% of the current massive SF activity in type T=3 and 4 galaxies occurs in the radial range of the outer peak. The excess SF is somewhat harder to estimate, but flattening the ``hump''
between 0.26 and 0.72 r24 reduces the overall SFR by
20%. This implies that inner (pseudo)-rings or other features
at the radial position of the end of the bar contain a significant
fraction of the total massive SF activity in the galaxies under
consideration. The equivalent number for the central peak is
10%.
We thus confirm that barred galaxies of T-types 3 and 4 have strong concentrations of SF both in their central regions (with the caveat that some part of the emission we base this statement on may be due to non-stellar processes) and in the regions at the ends of the bar.
5.3 Inner rings
The galaxy classifications in the Col. 3 of Table
4 show that four have an ``(r)'' classification,
indicating an inner ring, and five ``(rs)'' indicating a pseudo-ring.
This frequency is similar to or slightly higher than the fraction
expected from the numbers of such rings found in T= 3 galaxies by
Buta & Combes (1996). The radial size of the outer peaks found in the
present study is similar to that of inner rings, but the match is not
perfect. de Vaucouleurs & Buta (1980) give a formula for the radial size of inner
rings as a function of galaxy type and bar classification. For SBb
galaxies, this formula predicts inner rings to have radii 0.303
times the B25 isophotal radius, where the latter is very similar
to the R24 isophotal radius used in the present study. The
R-band outer peaks are centred on 0.38 times the R24 isophotal
radius, 25% larger than is expected for inner rings; and the
H
outer peaks are at somewhat larger radii still, 0.4-0.5 times
this isophotal radius. This implies that the SF we are seeing is not
directly associated with the bar ends, but is triggered in the spiral
arms lying just beyond this radius. In this context, it should be
noted that NGC 4548, one of the SBb galaxies in the present study, is
the prototype for the ``bracket type'' of barred galaxies
(Buta et al. 2002), where short spiral arms lie just outside the radius of the bar
ends, and ``overshoot'' the bar, rather than starting where the bar
terminates. In this galaxy at least, the SF causing the outer peak is
clearly located in these arm segments.
5.4 Bars and star formation
The impact of the presence of a bar on the total SFR in a galaxy has been studied by many authors (e.g., Martinet & Friedli 1997; Huang et al. 1996; Isobe & Feigelson 1992; Ryder & Dopita 1994; Hawarden et al. 1986; Pompea & Rieke 1990; Sheth et al. 2002; Aguerri 1999; Puxley et al. 1988; Tomita et al. 1996; Roussel et al. 2001; Verley et al. 2007; Dressel 1988), but no consensus has been reached as to whether the presence of bars causes a global enhancement of the SFR.
One of the most important characteristics of bars is that the non-axisymmetric mass distribution in their host galaxy can lead to the outward transport of angular momentum, and thus to the inward transport of gas (see, e.g., the review of Shlosman et al. 1990, for the theoretical view). The resulting enhanced central concentration of gas in barred galaxies as compared to non-barred galaxies has indeed been observed (e.g., Sakamoto et al. 1999; Sheth et al. 2005), although Komugi et al. (2008) note that varying Hubble type, indicative of the effect of the bulge, is more important for central gas concentration than the presence of a bar.
The central gas concentration caused by bars may lead to an enhanced SFR rate in bars, quite possibly in the form of circumnuclear rings of SF which can occur between the inner Lindblad resonances set up by the bar. Indeed, almost all of these nuclear rings occur in barred galaxies, and in the few that do not the influence of a past interaction can often be deduced (e.g., Knapen 2005). Whether nuclear starburst and AGN activity is, statistically, induced or facilitated by the presence of a bar is not a settled question, with theoretical studies supporting such a link (Wada 2004; Shlosman et al. 2000,1989; Wada & Habe 1995), whilst observations have hinted both in favour of (Laine et al. 2002; Hunt & Malkan 2004; Knapen et al. 2000) and against (Dumas et al. 2007; Martini et al. 2003; Laurikainen et al. 2004; Mulchaey & Regan 1997) this connection. A review by Knapen (2004) concludes that the evidence favours a slight effect connecting bars to both these types of activity, but the link is statistical and not direct, and subject to important caveats.
What has been studied perhaps less in the literature is what we present here: an analysis of the effect on bars on the radial distribution of current and past SF in disk galaxies. In addition, all past studies have considered radial profiles in surface brightness or equivalent, rather than the kind of flux profiles we present here, and which show much more clearly the effects of different components on the radial profiles. The classical study of observed bar properties, including radial surface brightness profiles, is that by Elmegreen & Elmegreen (1985), although before that other papers, such as those by de Vaucouleurs & Freeman (1972) and Elmegreen & Elmegreen (1980) had considered the Large Magellanic Cloud (LMC) and other late-type galaxies that resemble it. A common finding in all these, and subsequent, papers is that important areas of massive SF appear near the ends of the bar.
6 Conclusions
The spatial distributions of H

The central regions of the H
profiles of most galaxy types were
found to show a dip relative to the R-band profiles, which we
interpret as the signature of the older stellar populations in bulges
and bars, cf. disks. Surprisingly, this central dip is not present in
the mean light profiles of the earliest types studied here (T= 0-2), probably due to a combination of nuclear SF rings and
non-SF-related ENER emission in the central regions of many of these
galaxies. The mean H
and R-band profiles of the latest types
(particularly T= 10 Im types) show excellent agreement in overall
shape. There is no evidence for outer truncation of the mean H
profiles
relative to the R-band profiles for any type.
The mean H
profile for SAb, SABb and SBb (T= 3) galaxies showed a
central spike, and an outer peak at
0.45 r24. Both were
found to be characteristic features of barred T= 3 galaxies,
occurring in almost all of the individual galaxies of this type, and
were present at a lower level in the R-band mean T=3 profile, and
in the mean profiles for galaxies of T-types 4 and 5. The outer
peaks are at radii similar to or somewhat larger than those expected
for inner rings, and they constitute at least
20% of the total SF activity in the SBb galaxies. The central spikes
are resolved in most cases, implying that they are not purely powered
by AGN activity, and contribute
10% of the total H
flux.
Acknowledgements
The Jacobus Kapteyn Telescope was operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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Footnotes
- ... survey
- Based on observations made with the Jacobus Kapteyn Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.
All Tables
Table 1: Number of galaxies of each Hubble T-type and bar classification contributing to the mean profiles presented in this paper.
Table 2:
Peak and effective radii of the H
and R-band mean profiles,
in units of the R24 isophotal radius; and the mean value of this
radius, in kpc, all listed as a function of T-type.
Table 3: Properties of the central emission peaks.
Table 4: Properties of the outer emission peaks.
All Figures
![]() |
Figure 1:
The concentration index C30 plotted against
Hubble type T. The plot on the left shows the mean value of the
R-band C30 index for each type, with the error bars representing the
standard error on the mean for all
galaxies of that type. The plot in the centre shows the same mean
concentration indices for H |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
The difference in mean concentration index between
unbarred/weakly barred galaxies and strongly barred galaxies,
plotted against Hubble type T. For
points lying above the line, the unbarred/weakly barred galaxies
have more centrally concentrated emission than do their strongly
barred counterparts. The plot on the left shows the mean
difference in this index for the H |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Normalised mean profiles in H |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Peak and effective radii of the H |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Difference profiles, H |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Profiles made with the continuum light deliberately over- and under-subtracted (dot-dashed and dashed curves respectively), for five representative sample galaxies: UGC 4097 (Sa), UGC 4273 (SBb), UGC 6644 (Sc), UGC 6778 (SABc) and UGC 8490 (Sm). |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Mean R-band light profiles for T= 5 galaxies ( left) and T= 9 galaxies ( right), with the 6 profiles in each frame showing the bright, faint, large, small, far and near halves of the total sample from top to bottom. |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
Mean H |
Open with DEXTER | |
In the text |
![]() |
Figure 9: As Fig. 8 but showing the R-band profiles, again subdivided by bar classification. |
Open with DEXTER | |
In the text |
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