Astron. Astrophys. 331, 451-462 (1998)

4. The CO(1-0) emission of isolated and cluster galaxies

4.1. The CO(1-0) emission of isolated galaxies: a predictor

We now try to identify the relevant parameters which would allow to predict the CO(1-0) emission/molecular content of spirals. In order to study the effect of environment on the molecular gas content of galaxies, we have first to define and study a comparison subsample of isolated galaxies. To this purpose, we will use the isolated galaxies (labelled as ISOL), but since there are only 105 ISOL objects with HI and CO data, we will also include galaxies belonging to peripheral regions of clusters. Cluster galaxies have then been separated in two categories, CENTER and OUTSKIRTS, in a way that depends upon which cluster was considered. For Coma and A1367, we have used the aggregation parameter Agg (Gavazzi 1987): for Agg =1, 2, 3 or 4, the galaxies are considered as CENTER, while they are OUTSKIRTS for the other values of Agg. For Fornax and Virgo, whether a galaxy is CENTER or OUTSKIRTS depends upon its distance to the cluster center. The limit between the two regions has been varied from to , with no significant differences. In the following, this limit will be kept at . There are then 105 galaxies in the ISOL subsample (82 detected in CO(1-0) ) and 102 in the OUTSKIRTS one (81 detected), so that our reference sample amounts to 225 objects.

What are then the relevant parameters? It is clear that "size" and "form", which have a very good predicting power for HI (HG), are not sufficient for CO(1-0) . This can be suspected from inspection of Table 1: standard errors are systematically higher for than for HI. A third parameter seems therefore necessary. As it is widely known that the far-infared luminosity is a good predictor of the CO brightness of a galaxy, we have investigated and as the third parameter, but also, as a check, the blue luminosity and the blue surface brightness (we have also investigated the relationships with dynamical mass, but we have found that it is of much lower importance than and ).

Fig. 7 presents the variations of and with the blue luminosity and the blue surface brigthness. Fig. 8 presents the variations of the same quantities with the FIR luminosity and the FIR surface brigthness. As was already found by HG84, is almost independent of and very weakly dependent on (this is why they chose as the best predictor of the HI content). It is also independent of the FIR emission, which is a nice and expected behavior.

Fig. 7. Variation of the normalized gas masses and with the blue luminosity and the blue surface brigthness . There is little variation for the atomic phase and a significant one for the molecular phase.

Fig. 8. Variation of the normalized gas masses and with the far-infrared luminosity and the far-infrared surface brigthness . There is little variation for the atomic phase, but a strong one for the molecular component.

As for the molecular gas content, it shows a strong and significant variation with both the blue surface brightness and FIR luminosity, and an even stronger one with the far-infrared surface brightness . It is thus important to take the latter variation into account if one wants to predict the CO(1-0) emission of a galaxy with some accuracy, or to compare different samples.

We have found in the previous Section that there is a dependence of with morphological type. Part of it is likely due to the dependence of with type, which follows roughly the same pattern (see also Roberts & Haynes 1994).

We can then define an "expected" value of the normalized molecular hydrogen mass, (, using linear regressions: log(() = a(T) log() + b(T). T is the morphological type of the galaxy, or a group of morphological types (e.g. Scd-Irr), chosen in the same way as HG84. The values of a(T) and b(T) are given in Table 4.

Table 4. Values of the a(T) and b(T) coefficients, used to compute the expected value of the molecular content log((), from linear regressions of the form log( = a(T) log() + b(T). a(T) and b(T) are given for each morphological type bin. Survival analysis was used. Peculiar galaxies are not included in the sample, which contains only the ISOL and OUTSKIRTS catgories (see text).

4.2. The effect of environment on the CO(1-0) emission of galaxies

Now that we have a way to estimate the expected CO(1-0) emission of an "isolated" galaxy, we can try to evaluate the effects of the galaxy environment on their CO(1-0) emission/molecular gas content. To this aim, we define a CO deficiency parameter, CODEF, and another one, ENV, which describes the galaxy environment.

ENV contains four categories: ISOL, OUTSKIRTS, CENTER, and INTER. ISOL and OUTSKIRTS galaxies have been defined in the previous section. For cluster galaxies, we now have the CENTER category: all cluster objects that do not fall in the OUTSKIRTS category. The fourth group, INTER, gathers 23 galaxies which are known to be strongly interacting, such as ring galaxies (Horellou et al. 1995b). As a check, we have also computed the values of the HI deficiency HIDEF.

CODEF is computed using the results of the previous Section, in the following way:
CODEF = log( log(,
where ( and ( are respectively the observed and expected value of this parameter. With this definition, CODEF is positive when the galaxy is deficient in CO emission. Pec galaxies have not been considered in the previous Section; we will compare their CO emission with that of the reference sample described before, with no distinction of morphological type (last line in Table 4.1).

Mean values of CODEF and HIDEF are given in Table 4.2, while the probabilities that there are significant differences between the sub-samples are given in Table 6. As expected, CENTER galaxies are significantly HI-deficient, while OUTSKIRTS have the same mean (null) deficiencies than ISOLATED within the errors (this justifies the use of OUTSKIRTS galaxies as a reference sample for CENTER objects, see Sect. 4). On the other hand, we find no significant difference at the 5% percent level between any subsamples for their CO deficiency.

Table 5. Mean values of the CO deficiency, CODEF, and of the HI deficiency, HIDEF, for different galaxy environments ENV. CENTER galaxies are those belonging to cluster cores, OUTSKIRTS are found in the outer parts of clusters, ISOL are galaxies from the Karachenseva catalog, while INTER are interacting galaxies such as rings.

Table 6. Probabilities that galaxies in different environments have the same mean value of the CO deficiency (column 2) and HI deficiency (column 3). These mean values are given in Table 6. As expected, galaxies in cluster cores and outer regions have significantly different HI contents, core galaxies being HI-deficient and the others not. On the other hand, there is no significant variation of the CO deficiency with the galaxy environment, the mean value of CODEF being always consistent with 0.

We then conclude that in this sample, we see no sign of a modification in any sense of the CO emission of galaxies in the core of the Virgo, Fornax, Coma and A1367. This establishes on firmer grounds the conclusions drawn by Kenney & Young (1989) for the Virgo cluster, for the Coma supercluster by Casoli et al. (1991), and for the Fornax cluster by Horellou et al (1995b), which were all given in the absence of a reference sample.

As for the interacting galaxies in our sample, they do not seem especially rich in . This may seem in contradiction with the current wisdom that interacting galaxies are FIR-bright, and that FIR-bright galaxies are CO-bright. However, our definition of the normal CO emission/ content of a galaxy includes the far-infrared surface-brightness as a parameter. Moreover, interacting galaxies are not all FIR-bright, though the reverse may be true. CO surveys of optically-selected interacting galaxies have found that they have a low CO emission, that is, undetectable in many cases (Horellou & Booth, 1997), and Solomon & Sage (1988) have found that high mass galaxies are not all interacting. It is mainly the far-infrared emission that governs the CO emission, and not the presence of an interaction.

© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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