5 The dependence of $\vec{F_{\mathsf {15}}/F_{\mathsf 7}}$ on spiral and bar types and comparison with IRAS results

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 $(\Omega - \kappa / 2)_{\rm max}$ (where $\Omega$ is the gas circular rotation frequency and $\kappa$ 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.

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, $3\hbox {$^{\prime \prime }$ }$ 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 15$\mu$m and mostly affect the 7$\mu$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$\mu$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; ${\rm VCC}\,460 = {\rm NGC}\,4293$ stands between H I-deficient and H I-normal galaxies, and also has an early-type aspect. The case of ${\rm VCC}\,1326 = {\rm NGC}\,4491$ 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 25$\mu$m emission (normalized to the emission at 12 or 100$\mu$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 $F_{25}/F_{12} \simeq 2$. The same convention as in Fig. 2 applies for the representation of SA, SAB and SB classes. We have indicated the names of the Sy2 galaxy NGC4388 = VCC836 (see Sect. 7.1), and of the two galaxies with the lowest F15/F7 colors (note that NGC 6744 was not entirely mapped and that its integrated color is likely a lower limit).

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 25$\mu$m 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$\mu$m to 25$\mu$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$\mu$m than to the 12$\mu$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$\mu$m bandpass and contributes an ever increasing fraction.

The galaxies with a 15$\mu$m 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, $L_{\rm FIR}/L_{\rm B}$ spans the range [0.6 ; 7.3] with a logarithmic mean of 2.2 and dispersion by a factor 2.2, while $L_{\rm FIR}/L_{\rm B}$ 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$\mu$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$\mu$m excess and SA-SAB galaxies: the $L_{\rm FIR}/L_{\rm B}$ 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.