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

7. Conclusions

The following conclusions have been reached in this study of a large sample of disc galaxies observed both in HI and CO(1-0) :

1. In the sample spiral galaxies, there is more HI than , with only about one fourth to one third of the gas in molecular form in Sa-Sc spirals. This still holds if one considers the star-forming disc only, which has an ratio of about sixty percent,
2. This value of the molecular gas fraction has been obtained from CO(1-0) observations using a conversion factor X = 2.3 10²⁰. The previous conclusion is reinforced if, as suggested by recent ray and millimeter-wave continuum observations, the value of X for Sa-Sc spirals is actually lower,
3. We find a decrease of the molecular to atomic gas mass ratio for late-types, beginning at Scd's. This variation is due to the conjunction of two factors: the increase of the mass of gas in atomic form, and the decrease of the molecular gas amount. It is milder than what has been found in previous studies, late-type objects (Scd to Irr) having 1/10 of their gas in molecular form,
4. This decrease is actually related to the dynamical mass of the galaxy, and is absent for objects with . The reasons for this difference remain to be explored,
5. We give a recipe to estimate the molecular gas content/CO(1-0) emission of a galaxy knowing its size (), form (morphological type), and far-infrared emission (). This allows us to define a CO deficiency parameter, CODEF, analogous to what has been defined for the HI emission. Using this parameter, we show that galaxies in cluster cores, which are strongly deficient in their HI emission, are not CO-deficient.

© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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