6 The role of central regions

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 15$\mu$m flux originating from the central region (inside the radius $R_{\rm CNR}$) 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$\mu$m and the integrated F15/F7 color. Galaxies with no identifiable central regions, i.e. surface brightness profiles consistent with a single disk component, have been placed at a null ordinate.

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 $R_{\rm CNR}$, fitted on 7$\mu$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 ( $F_{15}/F_7 = 0.89 \pm 0.14$ for the $1 \sigma$ dispersion), whereas circumnuclear colors form a broader distribution extending towards high values ( $F_{15}/F_7 = 1.59 \pm 0.78$).

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 13$\mu$m 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$\mu$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.

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.

Figure 6: Fraction of total 15$\mu$m fluxes arising from the central condensation, as a function of morphological type. As in Fig. 4, galaxies with no identifiable central regions have been placed at a null ordinate. The central fraction of F7 fluxes, not shown here, has a very similar behavior, with only slightly lowervalues.

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 15$\mu$m 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.