A&A 372, 406-426 (2001)
DOI: 10.1051/0004-6361:20010497
H. Roussel1 - M. Sauvage1 - L. Vigroux1 - A. Bosma2 - C. Bonoli3 - P. Gallais1 - T. Hawarden4 - S. Madden1 - P. Mazzei3
1 - DAPNIA/Service d'Astrophysique, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France
2 - Observatoire de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 4, France
3 - Osservatorio Astronomico di Padova, 5 Vicolo dell'Osservatorio, 35122 Padova, Italy
4 - Joint Astronomy Center, 660 N. A'ohoku Place, Hilo, Hawaii 96720, USA
Received 14 December 2000 / Accepted 22 March 2001
Abstract
We study the mid-infrared properties of a sample of 69 nearby spiral
galaxies, selected to avoid Seyfert activity contributing a significant
fraction of the central energetics, or strong tidal interaction,
and to have normal infrared luminosities. These observations were
obtained with ISOCAM, which provides an angular resolution of the order
of
(half-power diameter of the point spread function) and
low-resolution spectro-imaging information. Between
5 and 18
m, we mainly observe two dust phases, aromatic infrared
bands and very small grains, both out of thermal equilibrium. On this
sample, we show that the global
F15/F7 colors of galaxies are very
uniform, the only increase being found in early-type strongly barred
galaxies, consistent with previous IRAS studies. The
F15/F7 excesses
are unambiguously due to galactic central regions where bar-induced
starbursts occur. However, the existence of strongly barred early-type
galaxies with normal circumnuclear colors indicates that the relationship
between a distortion of the gravitational potential and a central
starburst is not straightforward.
As the physical processes at work in central regions are in principle
identical in barred and unbarred galaxies, and since this is where the
mid-infrared activity is mainly located, we investigate the mid-infrared
circumnuclear properties of all the galaxies in our sample. We show how
surface brightnesses and colors are related to both the available molecular
gas content and the mean age of stellar populations contributing to dust
heating. Therefore, the star formation history in galactic central regions
can be constrained by their position in a color-surface brightness
mid-infrared diagram.
Key words: galaxies: spiral - galaxies: ISM - stars: formation - infrared: ISM: continuum - ISM: lines and bands
As the high frequency of bars in galaxies becomes more evident (e.g. Eskridge et al. 2000), and as new techniques emerge to both observationally quantify their strength (Seigar & James 1998; Buta & Block 2001) and numerically simulate them, their effects on their host galaxies are of major interest, and in particular, it is worth checking whether they are indeed very efficient systems to drive nuclear starbursts in spiral galaxies.
Numerous studies have dealt with the respective star formation
properties of barred and non-barred spirals, mostly in the infrared,
since this is the wavelength regime where starbursts are expected to be
most easily detectable. Yet conclusions derived from such studies
appear to contradict each other, partly because the different
selection criteria result in samples with a more or less pronounced
bias toward starburst objects. For instance, in the IR-bright sample
analyzed by Hawarden et al. (1986), an important fraction of SB and SAB
galaxies (respectively strongly barred and weakly barred spirals in the classification of de Vaucouleurs et al. 1991) shows a 25m emission
excess (with respect to 12 and 100
m) absent in the SA subsample
(non-barred spirals), which can be accounted for by a highly increased
contribution of Galactic-like H II regions to the total emission.
On the other hand, Isobe & Feigelson (1992), using a volume-limited sample and
performing a survival analysis to take into account the frequent IRAS
non-detections, found that the far-IR to blue flux ratio (
)
is rather independent of the bar class. The contradiction is marginal
since
does not give a direct estimation of the star
formation activity, especially when dealing with quiescent normal galaxies:
the blue light originates partly from young stars and, as Isobe & Feigelson (1992)
emphasize,
depends on the amount and spatial distribution
of dust with respect to stars. The relationship between the 25
m excess,
quantified by
F25/F12, and
in a galaxy sample with good quality data is indeed highly dispersed.
Huang et al. (1996) investigated the 25
m excess as a function of IR
brightness and reconciled the two previous analyzes: a significant
excess can occur only if
is larger than a threshold
value of
0.3. Therefore, a statistical effect of bars on star
formation can be demonstrated only in suitably selected samples.
Huang et al. (1996) also emphasized that the difference between barred and
unbarred spirals concerns only early types (S0/a to Sbc).
Studies of the infrared excess in barred galaxies mostly rest on the
integrated IRAS measurements, which do not allow the determination of the
nature and location of regions responsible for this excess. However,
dynamical models and observations at other wavelengths give evidence that
the infrared activity should be concentrated in circumnuclear regions
(see for instance the study of NGC5383 by Sheth et al. 2000). In addition,
high-resolution ground-based observations near 10m of galaxy centers
(Devereux 1987; Telesco et al. 1993) have shown that the dust emission is
more concentrated in barred galaxies.
Theoretically, bars are known to be responsible for large-scale redistribution
of gas through galactic disks. In a strong barred perturbation of the
gravitational potential, shocks develop along the rotation-leading side of
the bar and are associated with strong shear, as shown by Athanassoula (1992)
and references therein (also Friedli & Benz 1993).
They induce an increase of gas density which is traced by the thin dust
lanes widely observed in bars, producing a contrasting absorption of optical
light (Prendergast 1962, unpublished; Huntley et al. 1978). Due to these shocks,
gas loses angular momentum and flows towards the circumnuclear region. This
picture is confirmed by direct observations of inward velocity gradients
across bars in ionized gas lines, CO and H I
(e.g. Lindblad et al. 1996; Reynaud & Downes 1998; Mundell & Shone 1999). Regan et al. (1997) derive a
gas accretion rate of
into the
circumnuclear ring of NGC1530.
Statistical evidence is also found for higher gas concentrations
in the center of barred galaxies
(Sakamoto et al. 1999, who however observed only SABs, except NGC1530),
and for more frequent circumnuclear starbursts in barred
galaxies, as reported by Heckman (1980), Hawarden et al. (1986), Arsenault (1989)
(who, more exactly, found more probable starbursts in galaxies with both bar and
inner ring, supposed to be a signature of one or two inner Lindblad resonance(s)),
Huang et al. (1996), Martinet & Friedli (1997) and Bonatto et al. (1998).
Aguerri (1999) has moreover reported that
the global star formation intensity of isolated spirals (mostly of late types)
is correlated with bar strength as quantified by means of its projected axial
ratio, which is surprising in view of the very different timescales of bar
evolution (1Gyr) and star formation in kpc-scale regions
(
107-8yr). Indeed, Martinet & Friedli (1997), using carefully selected
late-type galaxies, found no such correlation, the bar strength being
quantified either by its deprojected axis ratio or its deprojected length
relative to the disk diameter. The fact that only a fraction of strongly barred
galaxies exhibit star formation excess (as evidenced by their IRAS colors)
is explained by these authors with numerical simulations of bar evolution
including gas physics. They show that a strong starburst occurs shortly after
bar formation and quickly fades away (in typically less than 1Gyr); meanwhile,
the strength and other properties of the bar evolve, but the bar remains strong
if it was initially strong. The existence of strongly barred galaxies in a
quiescent state is thus to be expected, presumably because the available
gas supply has been consumed in previous bursts.
This paper is aimed at characterizing the mid-infrared excess in barred
galaxies, with the possibility to carry out a detailed and systematic
spatial analysis due to the good angular resolution of ISOCAM (the
half-power beam diameter is less than
at 7
m), and
hence to locate unambiguously sites of enhanced infrared activity.
Although dust is a more indirect tracer of young stars than far-ultraviolet
ionizing radiation or optical recombination lines, the infrared
emission suffers relatively minor extinction effects, which are very
difficult to correct and hamper shorter wavelength studies.
In a companion paper (Roussel et al. 2001a, hereafter Paper II), we
have shown that in galactic disks, mid-infrared emission is a reliable
star formation indicator. Here, we concentrate on central regions of
galaxies where the dust heating regime is markedly different from that
in disks.
For this purpose, we have analyzed a sample of 69 nearby spiral
galaxies, imaged at 7 and 15m with the camera ISOCAM on board ISO
(described by Cesarsky et al. 1996c). We have also obtained low-resolution
spectroscopic information for a few galaxies, enabling us to identify and
separate the various dust components emitting between 5 and 18
m.
7
m images and
F15/F7 flux density ratios of selected regions,
together with optical images, are presented in Roussel et al. (2001b) (hereafter
the Atlas). For a description of data reduction and analysis, and a summary
of morphological properties of the sample, the reader is also referred
to the Atlas.
The sample is intended to be representative of normal quiescent spirals, and contains galaxies of moderate infrared luminosity. It covers three guaranteed time programs of ISOCAM. The first one (Cambarre) consists of nearby barred galaxies, the second one (Camspir) of a few large-size spirals of special interest (NGC1365, 4736, 5194, 5236, 5457 and 6744) and another subsample is drawn from the Virgo cluster sample of Boselli et al. (1998) (Virgo program), containing relatively fainter and smaller galaxies, both barred and unbarred. This sample was supplemented by comparable spirals in the ISOCAM public archive, from the programs Sf_glx (Dale et al. 2000) and Irgal (PI T. Onaka). All of the observations were reduced in the same way to form a homogeneous sample. The final set comprises 69 spiral galaxies at distances between 4 and 60Mpc. We have divided them into three main categories according to morphological classes in the RC3 (de Vaucouleurs et al. 1991): SBs (accounting for about half the sample with 37 galaxies), SABs (20 galaxies) and SAs (12 galaxies). The latter two classes are merged to form the control sample to compare with SB galaxies. This sample, although not statistically complete, has been selected according to the following requirements:
Despite the incompleteness of the sample, we have checked that it is
very similar to the magnitude-limited CfA galaxy sample
(Thuan & Sauvage 1992), from the point of view of its infrared brightness
normalized by blue starlight. For CfA spiral galaxies detected in all
4 IRAS bands and with blue magnitudes in the RC3, log(
)
falls in the interval [
-0.97 ; +0.98] with a mean value of 0.05. Using
the same IRAS references as those in Thuan & Sauvage (1992), i.e. in order of
preference Thuan & Sauvage (1992), Rice et al. (1988), Soifer et al. (1989) and
Moshir et al. (1989), galaxies in our sample have log(
)
in the interval [
-1.58 ; +1.67] with a mean value of 0.01. For that set
of references, a Wilcoxon-Mann-Whitney (WMW) test indicates that the
probability for the two populations to have the same
distribution is about 75%. We note that the IRAS 12
m fluxes
often disagree with our 7 and 15
m fluxes, although the bandpasses
overlap. Thus, when we use IRAS data, we take them from the references
we consider the most reliable (i.e. which provide the best match
between 12
m and our 7-15
m flux densities). In that case,
log(
)
falls in the interval [
-0.77 ; +0.95] with a
mean value of 0.02, and the WMW test gives a probability of about 40%.
Hence, our sample is not different from optically complete samples
regarding the fraction of the energy radiated in the infrared.
![]() |
![]()
a The H I deficiency according to the
definition and reference values of Guiderdoni & Rocca (1985). It is normalized by the
dispersion in the field sample. Diameters are taken from the RC2 for consistency,
and H I fluxes from the indicated references. NGC4567/68 are
unresolved in H I. When
![]() ![]() b The given range represents the effect of varying the scale length of the CO distribution from once to twice that of infrared circumnuclear regions (see text). b is the beam HPBW of the observations used. (*) NGC 4535 was observed by A. Bosma, D. Reynaud and H. Roussel at the IRAM 30m telescope. Signs of tidal interaction. ``doa'': asymmetrical distortion of outer arms. "mbs'': magellanic barred spiral. "am'': amorphous. "pc'': past collision. "note'': On DSS images, the brightness peak is displaced by ![]() ![]() ![]() ![]() ![]() # Classified SA in the RC3, here considered a SB after the morphological arguments of Phillips & Malin (1982) and McLeod & Rieke (1995), and the recent kinematic analysis of Veilleux et al. (1999). d The distances of ESO317-G023, NGC6753 and 6156 were assumed to be those of the galaxy groups LGG199, LGG426 and LGG407 (Garcia 1993); those of NGC5786 and 7771 were estimated from the H I redshift, that of NGC3620 from the CO redshift and those of NGC5430, 5937 and 6824 from the optical redshift (with h100 = 0.75). |
Table 1 lists some general characteristics of the galaxies. The morphological classification adopted is that of the RC3 (de Vaucouleurs et al. 1991). Although it is based on blue images, which may not be as appropriate as near-infrared images for detecting bars, many more galaxies are classified as barred in this catalog than for instance in Sandage & Bedke (1994). We have found only two galaxies classified as SA in the RC3 and possessing a bar (as described in the following). A drawback of using the SB and SAB classes of the RC3 is that they do not constitute a measure of the bar dynamical strength. The bar strength is however difficult to quantify, and reliable measures, such as those of Buta & Block (2001), are scarce. In the following, we will refer to bar lengths, normalized by the disk diameter, because longer bars are able to collect gas from inside a larger area and have low axis ratios, which are among the (unsatisfactory) quantities used to estimate bar strengths; bar lengths are in addition relatively easy to measure.
The two sub-samples of spirals found in the field or loose groups and Virgo galaxies have been separated, because they differ both in their aspect in the infrared (Virgo members are fainter and less extended) and in their environment. Although Virgo is not a very rich cluster, the interaction of central galaxies with the intracluster gas and with their neighbours is likely to cause either a depletion or an enhancement of star formation activity in the outer parts of disks and also to have global dynamical consequences. An extreme case is the galaxy NGC4438 (= VCC1043), whose very perturbed morphological appearence was successfully modelled by Combes et al. (1988) as the result of a collision with NGC4435. Several Virgo members have truncated H I disks due to the interaction with the cluster hot gas (Cayatte et al. 1990); a very clear example is NGC4569 (= VCC1690), which on optical photographs shows the juxtaposition of a bright and patchy inner disk structured by star formation sites and dust lanes, and a low surface brightness and very smooth outer disk with faint spiral arms. Severely H I-stripped galaxies can indeed be recognized in the optical as anemic (defined by van den Bergh 1976 as an intermediate and parallel sequence between lenticulars and spirals), due to the suppression of star formation where the gas density is too low. Table 1 also indicates whether signatures of nuclear activity or tidal interaction exist.
In addition to these, some galaxies deserve special comments (see the Atlas for more details) and should be considered cautiously in the interpretation of the data set:
All galaxies were observed with two broadband filters, LW3 (12-18m)
and LW2 (5-8.5
m), that we shall hereafter designate by their central
wavelength, respectively 15 and 7
m. This was expected to provide
F15/F7 colors directly linked with star formation intensity, since
the LW2 filter covers the emission from a family of bands (see
Sect. 4), which are ubiquitous in the interstellar medium,
and LW3 was supposed to cover mainly a thermal continuum observed to rise
faster than the emission bands in star-forming regions, for instance from
the IRAS
F25/F12 ratio (Helou 1986); however, we will see that
the picture is more complicated. Maps covering the whole infrared-emitting
disk were constructed in raster mode. In all cases, the field of view is
large enough to obtain a reliable determination of the background level,
except for NGC4736 and 6744. The pixel size is either
or
,
depending on the galaxy size. The half-power/half-maximum
diameters of the point spread function are respectively
/
at 7
m with a
pixel size,
/
at 7
m with a
pixel size,
/
at 15
m with a
pixel size and
/
at 15
m with
a
pixel size. The data reduction is described in the Atlas.
Since the emission from various dust species and atomic lines is mixed in the
broadband filters (see Sect. 4), it is essential to complement
our maps with spectro-imaging data. These allow an estimate of the
relative importance of all species as a function of the location
inside a galaxy. We have thus obtained spectra between 5 and 16m
of the inner disks (
or
)
of five bright galaxies: NGC613, 1097,
1365, 5194 and 5236 (Fig. 1). Spectra
averaged over a few central pixels covering approximately the extent
of the circumnuclear region (left column) are compared with spectra
averaged over the inner disk, excluding the central part and a
possible ghost image (middle column). The right column shows the
observed spectrum of the faintest pixels, consisting of the zodiacal
spectrum contaminated by emission features from
the target galaxy, because the field of view never extends beyond the
galactic disk. For this reason, we cannot measure exactly the level
of the zodiacal foreground to remove. Instead, as explained in the
Atlas, we first fit a reference zodiacal spectrum to the average spectrum
of the faintest pixels (excluding the spectral regions where emission
features appear). The upper limit to the zodiacal foreground
is set by offsetting the fitted spectrum within the dispersion range,
with the additional constraint that the corrected disk spectrum remains
positive; the lower limit is symmetric to the upper limit with respect to
the fit. This makes little difference for the nuclear spectra but it does
for the disk spectra, although it does not affect the spectral shape.
Note that due to the configuration of the instrument, two different filters
are used for the short and long wavelength parts of the spectra, and that
a small offset can result at the junction of these filters, around
9.2
m.
The mid-infrared maps generally show an intense circumnuclear source.
Decomposing surface brightness profiles into a central condensation
and a disk (see details in the Atlas), we define a radius for this
circumnuclear region,
.
Total fluxes and fluxes inside
are listed in Table 2 with the background level for each
broadband filter. Explanations about the method employed for photometry
and the estimation and meaning of errors can be found in the Atlas. The
dominant uncertainty arises from memory effects for relatively bright galaxies,
and from other sources of error (essentially the readout and photon noise)
for faint galaxies, especially at 15
m. For galaxies drawn from the
Sf-glx project, the number of exposures per sky position is very small
(
10) and does not allow a proper estimate of memory effects: their
photometric errors are thus especially ill-determined. Typical errors are
10% at 7
m and 18% at 15
m. Note that flux density
calibration uncertainties, which are of the order of 5 to 10%, are not included.
However, this is a systematic effect, hence not affecting relative fluxes.
![]() |
![]()
a The conversion from flux densities to fluxes
is:
![]() ![]() ![]() #: Nucleus saturated (for NGC5236: both at 15 and 7 ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() (-): The field of view is too small to allow a precise determination of the backgroud level and total fluxes are lower limits. The error bars are only formal. The comparison of our measurements with those of Rice et al. (1988) at 12 ![]() ![]() ![]() ![]() (+): From their spectral energy distributions shown by Boselli et al. (1998), these galaxies probably have a non-negligible contribution from the Rayleigh-Jeans tail of cold stars to their 7 ![]() ![]() |
The spectra shown in Fig. 1 are strikingly similar to one another. They contain some features also seen in spectra of reflection nebulae, atomic and molecular envelopes of H II regions, atmospheres of C-rich evolved stars as well as the diffuse interstellar medium. We can thus safely assume that the results obtained on these resolved Galactic objects can be readily extrapolated to the emission of galaxies where individual sources are no longer resolved.
The emission between 5 and 16m is dominated by the so-called
unidentified infrared bands (UIBs) at 6.2, 7.7, 8.6, 11.3 and
12.7
m. Our spectra also display weak features which have
previously been detected as broad features in SWS spectra of starburst
objects (Sturm et al. 2000) at e.g. 5.3, 5.7, 10.7, 12.0, 13.6, 14.3
and 15.7
m
. A 7.0
m
feature can tentatively be identified as an [Ar II] line
(6.99
m) or an H2 rotational line (6.91
m), but our spectral
resolution (
)
prevents a more definite
identification. We note however that the [Ar II] line has been
identified in the high-resolution SWS spectra of starburst galaxies
(Sturm et al. 2000).
It was originally proposed by Duley & Williams (1981) that UIBs are due to organic functional groups on carbonaceous grains. Léger & Puget (1984) instead favoured vibration modes of C-C and C-H bonds only, in large polycyclic aromatic molecules not in thermal equilibrium with the local radiation field (the so-called PAH model). The constancy of the spectral energy distribution of UIBs, regardless of the radiation field (Sellgren 1984; Uchida et al. 2000), implies an impulsive heating mechanism, where upon absorption of a single UV photon, the carriers undergo a very rapid and large temperature increase and then radiatively cool before the next absorption. Alternative candidates for the UIB carriers are various hydrogenated and oxygenated carbon grains, amorphous but partially ordered at the smallest scale (Borghesi et al. 1987; Sakata et al. 1987; Papoular et al. 1989), much similar to the idea of Duley & Williams (1981). Recent work by Boulanger et al. (1998b) indicates that UIBs are not due to molecules such as PAHs, but more likely to aggregates of several hundred atoms.
In the interstellar medium surrounding the OB association Trapezium
(Roche et al. 1989), the Orion bar (Giard et al. 1994) and M17 (Cesarsky et al. 1996a; Tran 1998),
these features are detected in the H II region and the
molecular cloud front (provided projection effects are minor), but the
emission peaks at the photodissociation interface (see also Brooks et al. 2000).
UIB carriers are likely destroyed in H II region cores, although the
estimation of the critical radiation field necessary to obtain a significant
reduction in UIB carrier abundance still remains to be done
(compare e.g. Boulanger et al. 1988; Boulanger et al. 1998a; Contursi et al. 2000).
![]() |
Figure 1: Spectra of central regions (left) and the inner disk (middle). The upper and lower limits are determined from limits on the zodiacal spectrum shown with dotted lines (right), adjusted using the average spectrum of the faintest pixels, also shown with its dispersion. The flux unit for all spectra is mJy arcsec-2. |
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While the 7m flux in spiral galaxies essentially consists of the UIB
emission, the 15
m filter covers the emission from mainly two dust
species: the hot tail of a continuum attributed to very small grains (VSGs)
of the order of 0.5-10 nm in size and most often impulsively heated like
UIB carriers (Désert et al. 1990), and also UIBs. The red wing of the
11.3
m band contributes little, but the band at 12.7
m and the
emission plateau that connects it to the 11.3
m band can be important;
the smaller UIB features listed above also contribute, although to a lesser
extent. When spatial resolution is high enough, the emission from
VSGs and UIB carriers can be clearly separated: around M17 and in the
reflection nebula NGC7023, the VSG continuum strongly peaks in a
layer closer to the excitation sources than the UIBs, inside the ionized
region for M17 (Cesarsky et al. 1996a, 1996b). Therefore, the
F15/F7flux ratio decreases with increasing distance from the exciting stars
of an H II region.
In the spectra of all five galaxies (Fig. 1), the
intensity ratios of UIBs are remarkably stable, which is a common property
of a variety of astronomical sources (Cohen et al. 1986; Uchida et al. 2000). The only
highly varying feature is the VSG continuum that is best seen longward
of 13m. It has various amplitudes and spectral slopes in galactic
nuclei. It remains very modest compared to that in starburst galaxies
(Tran 1998; Sturm et al. 2000), and is hardly present in averaged disks.
In Paper II, we show that the integrated mid-infrared luminosity
of normal spiral disks is dominated by the contribution from
photodissociation regions (where the UIB emission is maximum). From a
comparison with H
luminosities, we show that this predominance of
the photodissociation region emission results in the fact that,
when integrated over the disk, the UIB emission is a good tracer
of massive young stars.
Finally, as alluded to earlier, a number of fine-structure lines can be
present in the mid-infrared spectral range, although their contribution
to the broadband flux is always negligible in spirals. In normal galaxies,
the most prominent is the [Ne II] line at 12.81m, which at
the spectral resolution of ISOCAM is blended with the UIB at 12.7
m.
No lines from high excitation ions such as [Ne III] at
15.56
m are convincingly detected, and the [Ne II] line
at 12.81
m is weak, since the intensity of the blend with the UIB at
12.7
m, relative to the isolated UIB at 11.3
m, is rather stable
in different excitation conditions. Some variation however exists.
To compare the strength of the [Ne II] line in our
galaxies to that observed by Förster-Schreiber et al. (2001) in the starburst galaxies
M82, NGC253 and NGC1808, we have measured in a similar way the flux
of the blend F12.75 above the pseudo-continuum drawn as a
straight line between 12.31 and 13.23
m, and the flux of the
11.3
m UIB F11.3 with its respective continuum level defined
in the same way between 10.84 and 11.79
m. We find that the energy
ratio
F12.75/F11.3 of circumnuclear regions decreases from
0.67 in NGC1365 to 0.60 in NGC613 and 5236, 0.52 in NGC1097 and
0.47 in NGC5194; in the averaged inner disks of NGC1365, 5236 and
5194, where it is still measurable, it takes the approximate values
0.5, 0.45 and 0.4. These figures are much lower than those observed
in cores of starburst galaxies by Förster-Schreiber et al. (2001), where it can reach
1.7, and argue for a generally small contribution of [Ne II] to
the spectra. Adopting as the intrinsic
F12.7/F11.3 UIB energy
ratio the minimum value of
F12.75/F11.3 that we measure in our
spectra, i.e. 0.4, we obtain a maximum [Ne II] equivalent
width of 0.22
m in the nucleus of NGC5236. As for the UIBs, their
equivalent widths in disks and central regions range respectively
between
m and
m
(these numbers do not take into account broad UIB wings that occur if
the bands are described by Lorentzians). Our estimates give circumnuclear values for
between 1.5 and 5.7, that
we can compare with the results of Sturm et al. (2000) in the starburst
galaxies M82 and NGC253 from their ISOSWS spectra with a high spectral
resolution (
,
versus
40
for ISOCAM). They obtain values of 0.96 and 1.32. The contribution
from the [Ne II] line to our spectra is thus confirmed to be
negligible with respect to starburst galaxies.
In their infrared analysis, Huang et al. (1996) pointed out that bars are
able to significantly enhance the total star formation only in
early-type galaxies (mixing all types between S0/a and Sbc). It is
also known that bars do not share the same properties all through the
Hubble sequence: among early types, or more exactly in spirals with
large bulges, since the relationship between Hubble type and bulge to
disk ratio is far from direct (e.g. Sandage & Bedke 1994; Seigar & James 1998),
they tend to be longer (Athanassoula & Martinet 1980; Martin 1995),
and their amplitude, with respect to that of the
underlying axisymmetric potential, tends to be higher. For instance,
Seigar & James (1998), using K band photometry to trace the stellar mass,
find that galaxies with the strongest bars have bulge to disk mass
ratios between 0.3 and 0.5. For larger bulges, their number of
galaxies is too low to derive any meaningful bar strength
distribution. Early-type bars host little star formation, except
near their ends and at their center, whereas late-type galaxies
generally harbor H II regions all along the bar (García-Barreto et al. 1996),
which suggests that their shocks are not as strong as in early types
(Tubbs 1982). Inner Lindblad resonances between the gas and the
density wave, which appear when there is sufficient central mass
concentration and when the bar rotates more slowly than
(where
is the gas circular rotation
frequency and
the epicyclic frequency), and which presence
induces straight and offset shocks along the bar
(Athanassoula 1992), are also typically expected in early-type
galaxies. These structural differences have consequences on the
efficiency of bars to drive massive inward gas flows.
![]() |
Figure 2: a) Integrated mid-infrared color F15/F7 as a function of morphological type for unbarred or weakly barred galaxies, represented respectively by open circles and crossed circles. Virgo galaxies are identified by their VCC number (see Table 1) and others by their NGC number. b) Same as a) for strongly barred galaxies. |
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We show in Fig. 2a the distribution of
F15/F7according to morphological type (as given in the RC3) for the control
subsample including only SA and SAB galaxies. For this population -
excepting NGC4102 -, the
mid-infrared color is remarkably constant around a value of 1 (ranging
from 0.7 to 1.2). This is rather typical of the color of the surface
of molecular clouds exposed to radiation fields ranging from that
observed in the solar neighborhood to that found in the vicinity of
star-forming regions.
F15/F7 colors observed toward H II
regions are typically of the order of 10, while those of photodissociation
regions range between 2 and the H II region values (Tran 1998).
The fact that
F15/F7 remains of the order of 1 in most galaxies -
it also shows generally little variation from pixel to pixel in disks
- indicates that, at our angular resolution, emission from H II
regions and their immediate surroundings is diluted by the
larger neighboring interstellar medium (at a mean distance of 20Mpc,
represent 300pc). In fact, in the Atlas, we show that
even in giant star-forming complexes that can be identified in the
maps,
F15/F7 rarely exceeds 2-3.
The case of strongly barred spirals is more complex
(Fig. 2b): whereas many of them share the same
integrated colors as their unbarred counterparts, an important
fraction shows a color excess, the maximum color being above 2.5 instead of 1.2 for SA(B)s. Furthermore, such an excess occurs only
among the earliest morphological types, from SB0/a to SBb.
Note that in bulges, the envelopes of K-M stars can contribute an
important fraction of the mid-infrared emission. However, this would
be negligible at 15m and mostly affect the 7
m band:
correcting for such an effect would only re-inforce the observed
trend. We also qualify that observation by noting that two galaxies,
NGC1022 and NGC4691, have
likely experienced a merger; gas may therefore have sunk to the center
as a result of the violent energy dissipation in the merger, and not
simply under the influence of the bar, which actually may have been
formed during the interaction. Dismissing these two objects however
does not change the fact that the color distribution of the strongly
barred galaxies shows 15
m excesses that are absent from that of
weakly barred or unbarred spirals.
One can wonder whether cluster galaxies introduce a bias in our sample, because a number of them are perturbed by their environment and thus may have an uncertain morphological type. Koopmann & Kenney (1998) have shown that a significant fraction of early-type spirals in Virgo have been "misclassified'' due to their dearth of star formation in the disk. The degree of resolution of spiral arms into star formation complexes is indeed one of the three criteria defining the Hubble sequence, but it is not unambiguously linked to the bulge to disk ratio. Concerning several Virgo members of our sample, the bulge is very small for the attributed type (Sandage & Bedke 1994), in such cases defined mostly by the disk appearance. This is of course related to the anemia phenomenon, due to gas deficiency caused by interaction with the intracluster medium. Of our Virgo galaxies of types S0/a-Sb, 10/14 are H I-deficient, versus 3/9 for types Sbc-Sdm (see Table 1, where def > 1.2 has been adopted as the criterion for H I deficiency). This apparent segregation with morphological type certainly results from the above classification bias. Thus, differentiating galaxies in Fig. 2 according to their true bulge to disk ratio would cause an under-representation of SA-SAB early-type spirals, which make the crucial part of our comparison sample. If we had to discard completely the early-type SA-SAB subsample, the maximum allowed conclusion from Fig. 2 would be that we observe a color excess in a fraction of early-type strongly barred galaxies, without excluding the possibility of such an excess in early-type non-barred galaxies, in which case another mechanism for mass transfer would have to be thought of.
However, at least five early-type SA-SAB spirals remain which are not
H I-deficient and thus unlikely to suffer from the above bias,
namely VCC92 = NGC4192, NGC3705, NGC4736, NGC5937 and NGC6824.
We do not consider NGC3885, SA0/a in the RC3, because it looks
like a genuine barred galaxy: its bulge is elongated in a direction
distinct from the major axis of outer isophotes and crossed by dust
lanes; it is furthermore classified as such by Vorontsov-Velyaminov & Arkhipova (1968) and
Corwin et al. (1985). These galaxies show no global color excess, like the
rest of the SA-SAB subsample, and like a number of bona-fide early-type SB
galaxies with normal H I content. Hence, our view should not be
too strongly distorted by the classification bias. We have also
checked the influence of this bias on H I-deficient barred galaxies
with a color excess. On optical images, VCC836 = NGC4388
unambiguously resembles classical early-type spirals, with a prominent
bulge crossed by thick dust lanes;
stands
between H I-deficient and H I-normal galaxies,
and also has an early-type aspect. The case of
is not
that clear, because it is a low-mass galaxy.
Our sample thus confirms and extends to the ISOCAM bands a phenomenon
that was evidenced from IRAS observations by Hawarden et al. (1986) and
Huang et al. (1996), namely that a significant fraction of SB galaxies can
show an excess of 25m emission (normalized to the emission at 12
or 100
m) compared with SA and SAB galaxies. The case of SAB
galaxies is in fact unclear: some of them show such an excess
according to Hawarden et al. (1986), but they are indistinguishable from SAs
in the analysis of Huang et al. (1996). From the present ISOCAM data, it
already appears that indeed SA and SAB galaxies share similar
mid-infrared properties.
![]() |
Figure 3:
Comparison of mid-infrared colors from ISO (
F15/F7) and IRAS
(
F25/F12). F25 always contains the VSG emission (see
Sect. 4) whereas at low temperatures, F15 is
dominated by UIBs, which explains the constancy of
F15/F7 below
a threshold of
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In order to compare more directly our results with IRAS-based results,
Fig. 3 shows the relationship between
F15/F7 and
F25/F12. For log
(F25/F12) < 0.3 (which
is close to the value given by Hawarden et al. (1986) as the limit for the
presence of a 25m excess),
F15/F7 shows no systematic variation
and SA, SAB and SB galaxies are well mixed. Above that threshold, SB
galaxies strongly dominate (the SAB galaxy with high colors is NGC4102)
and the
F15/F7 ratio follows the increase of
F25/F12.
Given the nature of dust components whose emission is covered by the
7
m to 25
m filters (Sect. 4), this behavior
can be explained as follows. The classical interpretation for the
variation of
F25/F12 is that it increases with the radiation
field due to the stronger contribution of VSGs to the 25
m than to
the 12
m emission, which collects mostly UIB emission
(Désert et al. 1990; Helou 1986). The fact that the
F15/F7 ratio
remains insensitive to the variation of
F25/F12 for
log
(F25/F12) < 0.3 implies that in this regime, VSGs provide
little flux to both ISOCAM bands as well. Past this threshold, the
increase of
F15/F7 signals that the VSG continuum has entered the
15
m bandpass and contributes an ever increasing fraction.
The galaxies with a 15m excess (
F15/F7 above 1.2, or 0.08dex)
also distinguish themselves from the rest of our sample by having on
average larger far-infrared to blue luminosity ratios. For this
subsample,
spans the range [0.6 ; 7.3] with a logarithmic
mean of 2.2 and dispersion by a factor 2.2, while
of the
complementary subsample falls in the interval [0.2 ; 9.0], has a
logarithmic mean of 0.9 and dispersion by a factor 2.4. However,
the 15
m-excess galaxies have far-infrared luminosities that are
equivalent to those observed in the rest of the sample. Hence, in
these galaxies with a VSG emission excess, a higher fraction of the
total emission is reprocessed in the whole infrared range. There is
also a slight difference, although not statistically significant,
between SBs with no 15
m excess and SA-SAB galaxies: the
logarithmic means and dispersion factors are 1.1 and
2.7 for SBs with no excess, and 0.8 and 2.2 for SAs-SABs.
That mid-infrared color excesses occur only in SB galaxies indicates that somehow, a global increase of the interstellar radiation field intensity is linked to the presence of a strong bar, although this condition is clearly not sufficient. The fact that many barred galaxies earlier than SBb appear very similar in their integrated color to their unbarred counterparts means that no simple link exists between the bar class, the bulge-to-disk ratio and the onset of a starburst in normal spirals. Several intervening parameters can be thought of: the true strength of the bar in dynamical terms (the separation into SB and SAB classes is subjective and too rough, and in a recent study, Buta & Block 2001 show that the SB class includes a wide range of actual bar strengths); the available gas content inside corotation; the star formation efficiency along bars and in central regions; the timescales for starburst activation and exhaustion; interaction with a companion or with the intracluster gas. Some of these effects can be investigated in the present sample. We will discuss them in Sect. 7, but first we turn our attention to mid-infrared properties of the central regions, as defined in Sect. 3 and in the Atlas.
As the presence of a bar is expected to influence the star formation in the circumnuclear region and much less in the disk (except in the zone swept by the bar), we are naturally led to emphasize the relative properties of nuclei and disks. Maps shown in the Atlas demonstrate that central regions, observed in the infrared, are prominent and clearly distinct from other structures, much more than on optical images.
In Fig. 4 we plot the fraction of the total
15m flux originating from the central region (inside the radius
)
as a function of the global
F15/F7 color. Galaxies for
which a central region could not be defined on the mid-infrared brightness
profiles are also shown, and are attributed a null central fraction. Galaxies
are not distributed at random in this plot, but rather on a two-arm
sequence that can be described in the following way: (1) high
F15/F7colors are found exclusively in systems where a high fraction of the
flux is produced in the circumnuclear regions; (2) galaxies with small
F15/F7 ratios (< 1.2) are found with all kinds of nuclear
contributions.
![]() |
Figure 4:
Relationship between the central flux fraction at 15![]() |
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The bar class appears to play a part in the location of galaxies in this diagram, although this is not clear-cut: all galaxies with high circumnuclear contribution (> 40%) and large F15/F7 colors (>1.2) are SB galaxies, apart from NGC4102, while SA-SAB galaxies are quite indistinguishable from one another and cluster in the small nuclear contribution (< 30%) and low F15/F7 color corner of the graph. There is also a clear preponderance of SB galaxies in all the centrally dominated range. Only two SA-SAB galaxies show very high concentration fractions, NGC3885 and NGC4102. The latter galaxy was already discussed; for NGC3885, strong indications exist that its bar class is incorrect (see the discussion in Sect. 5).
However, it is quite significant that SB galaxies cover both sequences in Fig. 4 and in particular are found all through the sequence of varying flux concentration and low F15/F7 color. Therefore, Fig. 4 shows that high global F15/F7colors require that the flux concentration be high, and that the galaxy be SB, but none of these two properties is enough to predict that the global F15/F7 ratio will be high. To understand the importance of the flux concentration, let us first study separately the colors of central regions and those of disks.
Figure 5 compares the
F15/F7 distributions observed in
the disk and in the central regions of our galaxies (whenever the radius of the
central regions
,
fitted on 7
m brightness profiles, could
not be defined, the galaxy has been considered as a pure disk). These histograms
indicate that
F15/F7 ratios of circumnuclear regions are higher than those
of disks (and this is a systematic property, verified for each individual
galaxy except NGC4736 and 6744, whose central regions are dominated by old
stellar populations). Colors of disks are fairly constant and close to the
integrated colors of SA-SAB galaxies (
for the
dispersion), whereas circumnuclear colors form a broader distribution
extending towards high values (
).
The cause for this difference of colors can easily be seen in the spectra
of Fig. 1: in all spectra with sufficient signal-to-noise
ratio, the relative intensities
of the UIBs are almost unchanged from galaxy to galaxy, or from central regions
to disks. On the contrary, the level and spectral slope of the continuum seen
longward of 13m is highly variable and always stronger in the central
regions than in the disks. This continuum is attributed to VSGs (see
Sect. 3) and its presence in the 15
m band is a
characteristic sign of intense star formation (e.g. Laurent et al. 2000).
The reason why high global
F15/F7 colors require a high flux concentration
can be directly derived from Fig. 5: only the central
regions of galaxies are able to reach high
F15/F7 colors, and they have to
dominate the integrated emission to affect the global color. Furthermore,
the fact that the two color histograms overlap explains why a high flux
concentration does not necessarily imply a high
F15/F7 color.
![]() |
Figure 5: Compared histograms of F15/F7 colors averaged in disks and in circumnuclear regions. The galaxies used are respectively those whose disk is not strongly contaminated by the central component (the excluded galaxies are NGC1022, NGC4691, VCC1419 = NGC4506, NGC1326 and NGC3885), and those with central regions that could be adjusted on surface brightness profiles (otherwise the galaxy is considered to be composed only of a disk). The isolated galaxy with a very low central color is NGC6744, which is clearly devoid of young stars all inside its inner ring. |
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We however still have to identify the property or properties required, in addition to belonging to the SB class, for a galaxy to show a high mid-infrared flux concentration. We have seen in Fig. 2 that the morphological type plays a major part in the presence of high colors. Figure 6 shows the evolution of the concentration fraction as a function of morphological type. It confirms that for SB galaxies, there is a definite trend for the central flux fraction to rise as the morphological type gets earlier. More precisely, SB galaxies with central fractions greater than 40% are found predominantly among galaxies earlier than Sb.
It is less clear in Fig. 6 whether SA-SAB
galaxies follow a similar or a different trend, partly because of the
lack of such bar classes in our sample for types S0/a and Sa, and also
because types Sab and Sb may be incorrect due to the morphological
classification bias affecting cluster galaxies, as already discussed
in Sect. 5. In that section however, we emphasized
the existence of a set of five early-type SA-SAB spirals which do not
suffer from morphological misclassification: VCC92 = NGC4192, NGC3705,
NGC4736, NGC5937 and NGC6824. As apparent in
Fig. 6, they all have a low central flux
fraction, much lower than that observed in SB galaxies in the same
range of types. This supports the view that the trend seen for
increasing concentration fraction with earlier type concerns only SB
galaxies (or peculiar objects like VCC1043), SA-SAB galaxies having a
generally low concentration factor whatever their type.
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Figure 6:
Fraction of total 15![]() |
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We can summarize our findings in this section in the following way:
integrated
F15/F7 colors of galaxies are generally of the order of 1.
However,
F15/F7 is often higher in central regions.
Spiral galaxies with high
F15/F7 colors must simultaneously be (1)
dominated by their central regions, (2) of bar type SB, and (3) of
morphological type earlier than Sb. However, the reverse is not true: as
can be seen in Fig. 6, NGC5383 (a Markarian galaxy),
1672, 1365 and 1097 for instance fulfill these conditions - between 55 and
75% of their 15m radiation comes from small central regions (respectively
17, 8, 6 and 8% of the optical diameter) - yet their
F15/F7 color is
very similar to that of disk-dominated galaxies. This suggests that they host
at their center larger concentrations of gas and dust than in the average
of galaxies of the same Hubble type, but for some yet undetermined reasons,
they presently undergo smooth star formation instead of a nuclear starburst.
We propose that either the net gas inflow rate to the center has decreased
(due to a slower replenishment from the inner disk which would have been
previously partially depleted in gas, or a smaller efficiency of the evolved
bar to make gas lose its angular momentum) or, since star formation bursts
occur on a much shorter timescale than bar life, that we are imaging
these objects at a period of quiescence in-between bursts.
Concerning this last point, see the results of the simulations of
Martinet & Friedli (1997) and the population synthesis estimates of
Kotilainen et al. (2000) for the circumnuclear rings of NGC1097 and 6574.
Of the SB galaxies, four are known to host a Seyfert nucleus: in order of
decreasing flux fraction from the central condensation, VCC836 = NGC4388,
NGC1365, NGC1097 and NGC1433. For these, the high central color could arise
from dust heated by non-stellar radiation from the accretion disk and halo of the
central object and would thus not necessarily indicate the presence of massive
stars. For NGC1097, we have the direct visual evidence that the contribution from
the active nucleus to the circumnuclear emission is negligible, since the central
mid-infrared source is resolved into the well-known star-forming ring, which is
very bright, and a faint point source at the nucleus. Correcting the images
of NGC1097 for dilution effects with a procedure analog to CLEAN (see the Atlas
for more detail), we obtain fractions of the total circumnuclear fluxes contributed
by the nuclear point source of less than 3% at 7m and about 1% at 15
m.
This central source was measured inside a radius of
,
while the ring
extends between radii
and
.
We can also inspect the low-resolution spectra between 5 and 16m of
the central regions of NGC1097 and 1365 (left column of
Fig. 1). Indeed, Genzel et al. (1998) and Laurent et al. (2000) have shown
that a strong continuum at 5
m and small equivalent widths of the UIBs are
signatures of dust heated by an active nucleus. Yet all our spectra are
similar to that of the inner plateau of NGC5194 (
in diameter) - which also contains a weak Seyfert nucleus, but completely
negligible - and to that of NGC5236: they are dominated by UIBs in the
5-10
m range and the underlying
continuum at 5
m is comparatively very low. We conclude that in these
galaxies, the contribution of non-stellar heating to the emission observed
inside
is small.
The cases of NGC4388 and NGC1433 can only be discussed on the basis
of imaging results. The central condensation of NGC1433 is large (we
have determined a diameter of
kpc) and
extremely smooth, much flatter than the point spread function: we
therefore consider unlikely a major contribution from the
LINER/Seyfert nucleus, which should manifest itself as a point
source. For NGC4388, we cannot conclude and the active nucleus may
be dominant. We can only mention that its global color is lower than that
of VCC1326 = NGC4491, and this is marginally true as well for the
nucleus, and that the nucleus of VCC1326 is not classified as
active
.
Hence, the presence of Seyfert nuclei does not modify our interpretation that high mid-infrared colors in the present sample are not due to dust heated by non-stellar photons and should rather signal the existence of central starbursts.
We now examine the most likely cause of the 15m emission excesses
detected in our sample, central starbursts triggered by the bar
dynamical effects. We warn that NGC1022 and NGC4691 should be
considered apart: their dust emission comes almost exclusively from
central regions of
1kpc, but this is more likely due to a
past merger than to the influence of the bar, which may have been
formed or transformed simultaneously as the starburst event was
triggered.
To see if a significant difference exists between the central molecular gas content of circumnuclear starburst galaxies and quiescent ones, we have searched the literature for single-dish CO(1-0) data in the smallest possible beams. Single-dish data are better suited to our purpose than interferometric data since the latter are scarcer and do not collect all the emission from extended structures. The conversion of CO antenna temperatures to molecular gas masses is approximate for two main reasons: the H2 mass to CO luminosity ratio varies with metallicity and physical conditions; and the derivation of CO fluxes requires the knowledge of the source structure, because it is coupled to the antenna beam to produce the observed quantity which is the antenna temperature.
Sensible constraints on the structure of CO emission can be drawn from
that observed in the mid-infrared. As dust is physically
associated with gas, the mid-infrared emission spatial distribution
should follow closely that of the gas, but be modified by the
distribution of the star-forming regions that provide the heating, and
which are likely more concentrated than the gas reservoir. Since
Gaussian profiles provide an acceptable description of most infrared
central regions at our angular resolution, we have therefore assumed
that the CO emitting regions are of Gaussian shape, with half-power
beam width (HPBW) between one and two times that at 7m. The 7
m
HPBW were derived by matching Gaussian profiles convolved with the
point spread function to the observed 7
m profiles
.
To find the meaning of various antenna temperatures (with various corrections)
and which conventions are used in the literature, the explanations of Kutner & Ulich (1981)
and Downes (1989) were of much help. We converted given temperatures to the
scale
. We then attempted a
correction of antenna to source coupling, assuming a Gaussian source and a
Gaussian
diffraction pattern with angular standard deviations
and
.
The relationship below follows for the source brightness temperature
,
which is averaged over the beam in the observation, whereas we want to recover
its intrinsic value over the source extent:
![]() |
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= | ![]() |
|
![]() |
Although the beam of CO observations is in general larger than
,
it remains (except for NGC337) smaller than the diameter of the bar which
collects gas from inside corotation, believed to be located close to the end of
the bar (Athanassoula 1992), so that it is still meaningful to compare our
measurements on infrared condensations to CO data.
![]() |
Figure 7:
a) 7![]() ![]() |
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Figure 7a shows the variation of the 7m surface brightness
as a function of the average molecular gas surface density inside
.
Higher densities of the molecular material are associated with an increase in the
infrared brightness of the central regions. This is expected, since the amount of
dust scales with that of gas, which essentially consists of the molecular phase in
central regions of galaxies. More interesting is Fig. 7b where
we show the evolution of the
F15/F7 color inside
as a function
of the same quantity as in Fig. 7a. For the majority of our sample,
F15/F7 tends to rise, within a very large dispersion, when the molecular gas
mean density increases (it roughly doubles when the H2 surface brightness
varies by 1.2dex). However, a few galaxies dramatically depart from this trend:
for colors higher than 2.5 (log
F15/F7 > 0.4), there is a reversal in the sense
that hot circumnuclear regions seem to be depleted in molecular gas, with respect
to the normal H2 content-color distribution.
Although one can think of several reasons why their molecular content may be
underestimated (the standard conversion factor may not apply for these galaxies
due to their starburst nature or possibly due to a lower metallicity), it is
unlikely that this is the case. First, the implied underestimation factors
appear quite large, at least 4 to 10. Second, if we were to correct the
H2 masses by these factors to bring the galaxies within the trend observed
in Fig. 7b, then these objects would become abnormal in
Fig. 7a, with a deficit of 7m emission
.
The four deviating galaxies do not share a common property which would make them
special with respect to all the others. NGC4519 and IC1953 are similar SBd
galaxies, VCC1326 = NGC4491 is a small and low-luminosity SBa,
and VCC836 = NGC4388 is an edge-on Seyfert SBab (for which the molecular
content may be ill-determined due to the integration of the CO line throughout
the disk).
We thus propose the following interpretation for the galaxies that wander off the main trend in Fig. 7b: the main distribution corresponds to galaxies where the central starburst is more and more intense, as indicated by the high gas surface densities and colors. Galaxies at the turnover of the sequence may be observed in a phase of their starburst (not necessarily common to all galaxies) when it has consumed or dispersed most of the accumulated gas, because of a higher star formation efficiency. This suggests an interesting analogy with H II regions, for which the distinction between `ionization-bounded'' and "density-bounded'' is made (see Whitworth 1979, also for a discussion of the efficiency of molecular cloud dispersal by young stars). Dust should then be depleted too; however, because of the presence of massive stars, the remaining dust is exposed to a very intense radiation field and reaches a high F15/F7 color. This ratio may also increase due to the fact that the dust which was mixed with rather dense molecular clouds, of low F15/F7 color, has been dispersed too. Alternatively, the concentrations of molecular gas in these galaxies may be more compact than in the others and diluted in our large beam (we cannot exclude that the mid-infrared distribution includes an unresolved core which dominates the color). A confirmation of the above scenario clearly requires better measurements of the central gas content and high-resolution characterization of the starbursts.
Leaving the four galaxies in the upper left quadrant of Fig. 7b apart, the data support an interpretation in terms of starburst with standard properties: the infrared activity in galactic centers can be stronger when the available molecular gas is denser.
Figure 8 indicates how the
F15/F7 color inside
varies with the 15
m surface brightness in the same aperture. In principle,
the mid-infrared surface brightness can increase either because the amount of dust
in the considered area is higher (such as observed in Fig. 7a),
or because the energy density available to heat the dust increases. The trend
for higher
F15/F7 ratios at large 15
m surface brightnesses seen in
Fig. 8 indicates that indeed, the increase of the 15
m
surface brightness is at least partly due to rise of the mean energy density
in the CNR. In this diagram again, the galaxies with a peculiar behavior in
Fig. 7b stand apart, well above the locus defined by the least
absolute deviation fit
(dashed line). This supports the fact that
the trend seen in Fig. 7b is not due to an underestimation
of the H2 content, and lends further credit to the interpretation presented
in Sect. 7.2.1.
![]() |
Figure 8:
Variation of the mid-infrared color with the 15![]() ![]() |
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Another study by Dale et al. (1999) has already dealt with the joint variations of mid-infrared surface brightnesses and colors. However, contrary to Fig. 8 where the surface brightnesses and colors are those of the same physical region (the CNR) in a large sample of galaxies, in the Dale et al. (1999) study, resolution elements inside the target galaxies are first binned according to their surface brightness before the mean color of the bin is computed. As a result, a bin does not correspond to a physical object. We simply note that if galactic central regions are binned by surface brightness in Fig. 8, then the obtained mean locus is comparable to those shown by Dale et al. (1999).
The galaxies with the highest central
F15/F7 colors (
F15/F7 > 2.5) and
which stray from the main trend are barred, but their bars are of moderate lengths
(once deprojected and normalized by the optical diameter). In NGC4519, NGC4102,
VCC1326 = NGC4491 and IC1953, for which it could be estimated,
-0.3, when this ratio ranges between 0.06 and
0.67 in galaxies with measurable bar length. This confirms that the central
activity, signalled by a high
F15/F7 color, is not an increasing function of
bar strength, as can be expected from the different timescales for star formation
and bar evolution.
Since the bar strength alone is not sufficient to explain the observed
mid-infrared colors, and since the observational uncertainties are much smaller
than the scatter present in Fig. 8, one may suspect that part
of this scatter is due to intrinsic properties of each of the circumnuclear
starbursts considered. Indeed, given that mid-infrared emission likely traces star
formation on timescales longer than, for instance, recombination lines, it is
reasonable to expect that for similar mid-infrared brightnesses (corresponding
to similar gas and energy densities), the mid-infrared color could vary as a
function of the age of the stellar populations responsible for dust excitation.
Since star formation does not happen instantaneously all through a
1kpc region and likely occurs in cycles triggered by instabilities,
these stellar populations are multiple and their ages should be weighted to
reflect the successive generations of stars contributing to dust heating.
Using the population synthesis results of Bonatto et al. (1998), based on ultraviolet
spectra between 1200 and 3200Å, we can estimate the mean stellar age in the
central
,
weighted by the fraction of luminosity
emitted at 2650Å by different population bins. This was possible
for eleven galaxies of our sample in common with the sample of Bonatto et al. (1998).
We compare in Fig. 9 this mean age to the
F15/F7 color
deviation, defined as the difference between the observed color and
that predicted by the mean distribution of all galaxies (indicated by the least
absolute deviation fit in Fig. 8) at the same 15
m
surface brightness. For this purpose, we performed the mid-infrared photometry
in a slit aperture identical to that used by Bonatto et al. (1998). We indeed see that
the younger the weighted age, the higher the central
F15/F7 color deviation.
This is thus in agreement with our hypothesis
that much of the color variations in Fig. 8 may be due to age
variations of the exciting populations.
There are grounds to think that some scatter in Fig. 9 is due to the
methodology adopted by Bonatto et al. (1998) in their study. They have grouped galaxies
of their sample according to spectral resemblance, morphological type and
luminosity, and co-added all UV spectra of each group in order to increase the
signal to noise ratio before performing the population synthesis. However, it may
not be fully justified to average spectra of different galaxies with the same
overall shape but different spectral signatures. A further drawback of this
study is that it cannot properly take into account extinction, because of
the limitation
to a small spectral range in the UV: the derived very low extinctions are
meaningless. That is why the two galaxies departing from the well-defined trend
described above could owe their age to the method rather than to their
intrinsic properties:
![]() |
Figure 9:
The abscissa indicates the mean age of stellar populations, according
to the synthesis results of Bonatto et al. (1998), including the first six
elements of their base, stellar clusters to which they attribute ages
between 0 and 0.7Gyr for the first five and in the interval 0.7-7Gyr
for the last one, but excluding the oldest element, an elliptical bulge
representing ages between 7 and 17Gyr. The ages are weighted by the
fraction of the flux at 2650Å that each different population emits.
The contribution from continuous
star formation has been approximated by a constant flux fraction
(equal to the minimum value) and subtracted, in order to consider only
successions of bursts. The ordinate is the difference between the
measured
F15/F7 color and the color expected from the mean
relationship between central surface brightnesses and colors shown in
Fig. 8. For this graph, the photometry was performed
inside the same apertures as in Bonatto et al. (1998),
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Combining this result with that of Sect. 7.2.1, we can form the following sketch of what determines the mid-infrared properties of circumnuclear regions: the central surface brightness is connected to the amount of gas, as expected if gas-to-dust ratios are relatively constant. However, accumulation of gas in the center allows the triggering of intense star formation, so that the interstellar radiation field increases, reflected in higher F15/F7 ratios. Figures 8 and 9 suggest then that deviations from this simple description can be related to the star formation history of the circumnuclear regions. On-going starbursts produce excess F15/F7 colors, while faded starbursts are associated with F15/F7 deficits.
Additional variation in mid-infrared colors may arise from differences in metallicity and in the compactness of the starburst, with consequences on the amount and nature of the dust, but this is out of the scope of the present study.
We have studied the mid-infrared activity induced by bars in a sample of 69 nearby spiral galaxies with infrared luminosities spanning a large range below the class of luminous infrared galaxies. We have found that:
Acknowledgements
We thank our referee, Louis Martinet, for his helpful remarks.The ISOCAM data presented in this paper were analyzed using and adapting the CIA package, a joint development by the ESA Astrophysics Division and the ISOCAM Consortium (led by the PI C. Cesarsky, Direction des Sciences de la Matière, CEA, France).