7 Origin of the circumnuclear infrared excess

   
7.1 Non-stellar activity

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 7$\mu$m and about 1% at 15$\mu$m. This central source was measured inside a radius of $3\hbox {$^{\prime \prime }$ }$, while the ring extends between radii $\approx$ $6\hbox {$^{\prime \prime }$ }$ and $12\hbox{$^{\prime\prime}$ }$.

We can also inspect the low-resolution spectra between 5 and 16$\mu$m 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$\mu$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 ($\approx$ $50\hbox{$^{\prime\prime}$ }$in diameter) - which also contains a weak Seyfert nucleus, but completely negligible - and to that of NGC5236: they are dominated by UIBs in the 5-10$\mu$m range and the underlying continuum at 5$\mu$m is comparatively very low. We conclude that in these galaxies, the contribution of non-stellar heating to the emission observed inside $R_{\rm CNR}$ 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 $31\hbox{$^{\prime\prime}$ }\approx 1.7$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.

7.2 Circumnuclear starbursts

We now examine the most likely cause of the 15$\mu$m 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 $\approx$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.

   
7.2.1 Available molecular gas

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 7$\mu$m. The 7$\mu$m HPBW were derived by matching Gaussian profiles convolved with the point spread function to the observed 7$\mu$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 $T_{\rm R}^*$scale. We then attempted a correction of antenna to source coupling, assuming a Gaussian source and a Gaussian diffraction pattern with angular standard deviations $\theta_{\rm S}$ and $\theta_{\rm B}$. The relationship below follows for the source brightness temperature $T_{\rm b}$, which is averaged over the beam in the observation, whereas we want to recover its intrinsic value over the source extent:

$$T_{\rm R}^* \times (\theta_{\rm S}^2 + \theta_{\rm B}^2) = T_{\rm b} \times \theta_{\rm S}^2.$$

Table 1 contains the beam width of the observations and the derived H₂ masses for the adopted references. A conversion factor $f = N({\rm H}_2)\, /\, I({\rm CO}) = 2.3\times10^{24}~ {\rm molecules\,m}^{-2}\,({\rm K\,km\,s}^{-1})^{-1}$(Strong et al. 1988) has been used to compute the mass as:

$$M_{\rm H_2}\, /\, (2\, m_{\rm H}) f \times \int T_{\rm b}\, {\rm d}V~ ({\rm K\,km\,s}^{-1}) \times 2\, \pi\, (\theta_{\rm S}\, D)^2$$ =

$$\times~ (1 - \exp(-\frac{1}{2} (\alpha_{\rm CNR}\, /\, \theta_{\rm S})^2))$$

where $m_{\rm H}$ is the hydrogen atom mass and D the distance (in m). In the above formula, we estimate the mass only inside the angular radius $\alpha_{\rm CNR}$used for the infrared photometry of circumnuclear regions. Only when the central regions are resolved and mapped is there no need to assume a brightness distribution. H₂ masses derived in this way are probably not more precise than by a factor three, including the dispersion of the factor f, but the dynamic range in the sample is still sufficient to allow a discussion of the results.

Although the beam of CO observations is in general larger than $\alpha_{\rm CNR}$, 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$\mu$m surface brightness as a function of the average H2 surface density, both inside the circumnuclear regions defined by mid-infrared photometry (CNR). The limits on H2 mass are not true error bars, but simply indicate the effect of varying the scale of the Gaussian distribution from once to twice that measured at 7$\mu$m (see text). b) F15/F7 color as a function of the average H2 surface density, both inside the CNR, with the same convention for error bars as in a).

Figure 7a shows the variation of the 7$\mu$m surface brightness as a function of the average molecular gas surface density inside $R_{\rm CNR}$. 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 $R_{\rm CNR}$ 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 7$\mu$m 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.

7.2.2 Color of the central concentration and age of the starburst

Figure 8 indicates how the F₁₅/F₇ color inside $R_{\rm CNR}$varies with the 15$\mu$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$\mu$m surface brightnesses seen in Fig. 8 indicates that indeed, the increase of the 15$\mu$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$\mu$m surface brightness (in mJyarcsec-2), both inside the same aperture centered on circumnuclear regions. The mean error bar is shown in the upper left corner. The average location of disks in the diagram in terms of average color and global surface brightness (the disk area being delimited by the blue isophote $\mu _{\rm B} = 25$ mag arcsec-2) would be at ( -2.2, -0.05). The dashed line represents the formal least absolute deviation fit, performed including all the galactic central regions (used to define the "color deviation'' in Fig. 9).

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, $D_{\rm bar} / D_{25} \approx 0.2$-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 $\approx$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 $10\hbox {$^{\prime \prime }$ }\times 20\hbox {$^{\prime \prime }$ }$, weighted by the fraction of luminosity emitted at 2650Å by different population bins. This was possible for eleven galaxies of our sample in common with the sample of Bonatto et al. (1998). We compare in Fig. 9 this mean age to the F₁₅/F₇ color deviation, defined as the difference between the observed color and that predicted by the mean distribution of all galaxies (indicated by the least absolute deviation fit in Fig. 8) at the same 15$\mu$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), $10\hbox {$^{\prime \prime }$ }\times 20\hbox {$^{\prime \prime }$ }$, and error bars show the effect of varying the slit orientation, which is not given by Bonatto et al. (1998).

-

VCC1690 = NGC4569 is assigned the same large weighted age as NGC7552 (their UV spectra are co-added), but has a higher color excess with respect to the mean distribution in Fig. 8. In fact, Maoz et al. (1998) detected very strong P Cygni absorption lines of high-excitation ions (C IV, Si IV, N V) characteristic of the winds of massive young stars (< 6Myr), and its spectrum between 1220 and 1590Å is nearly identical to that of the starburst NGC1741B.

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NGC1433 is co-added with NGC4102, which results in a small weighted age. Yet its color excess is much lower than that of NGC4102 and more comparable to that of NGC7552. There is some evidence that the extinction in the central regions of NGC1433 is much lower than in other galaxies: whereas available Balmer decrement measures indicate H$\alpha$ absorptions of the order of 2-3 in other nuclei, the decrement given by Diaz et al. (1985) for NGC1433 indicates A(H$\alpha$)  $\approx 0.9$. Even if the Balmer decrement is not a good extinction measure, it is instructive to compare values in different galaxies. We also notice that NGC1433 is the only one among strongly barred spirals which has an amorphous circumnuclear region, with no hot spot that would indicate the presence of massive stellar clusters.

To conclude, Fig. 8 is a strong indication that the mid-infrared emission in circumnuclear regions is influenced by successive episodes of star formation over relatively long periods of time: on the mean, the F₁₅/F₇ color is a sensitive function of the mid-infrared surface brightness, but this relationship is modulated by the mean age of the stellar populations. A strinking example of this is NGC4736. Its central mid-infrared brightness is in the high range, but its central F₁₅/F₇ ratio is low, in accordance with its large mean stellar age confirmed by Taniguchi et al. (1996). From optical population synthesis, they find that a central starburst occurred about 1Gyr ago in this galaxy, and that subsequent nuclear star formation has proceeded at a low rate.

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.