From a CO survey of more than 300 galaxies, it has been concluded that the average molecular content was comparable to the atomic content: M(H2) / M(HI) ~ 1 (Young & Knezek 1989; Young & Scoville, 1991). But most of galaxies in this survey were selected from their IRAS flux, and this could introduce a bias. A recent survey by Casoli et al (1998) near the Coma cluster has shown an average M(H2) / M(HI) ~ 0.2.
2.1. Variation with morphological type
It is well established that the HI component is proportionally more abundant relative to the total mass in late-type galaxies. The opposite trend is observed for the H2, at least as traced by the CO emission. M(H2) / M(HI) is therefore smaller for late-types, by a factor ~ 10. However, this could be entirely a metallicity effect. Since the metallicity is increasing with the mass of the galaxy, a test is to select the most massive galaxies of late-type. For these high-mass galaxies, there is no trend of decreasing H2 fraction with type (Casoli et al. 1998).
2.2. Dwarf and LSB galaxies
The strong dependency of the H2 / CO conversion ratio on metallicity Z is also the main problem in the observations of dwarf and Low Surface Brightness (LSB) galaxies. Both have low metallicity. It appears that the conversion factor X can vary linearly and even more with metallicity, as predicted by Maloney & Black (1988). Not only, the low abundance of C and O lowers the abundance of CO, but also the dust is less abundant, and therefore the UV light is less absorbed, and spread all over the galaxy, photo-dissociating the CO molecules. When the dust is depleted by a factor 20, there should be only 10% less H2, but 95% less CO (Maloney & Black 1988).
In dwarf galaxies, CO emission is very low, and it is difficult to know the H2 content. If the HI / H2 ratio is assumed constant from galaxy to galaxy, then X varies with Z-2.2 (Arnault et al 1988). Recent results by Barone et al (1998), Gondhalekar et al (1998) and Taylor et al (1998) confirm this strong dependency on metallicity, increasing sharply below 1/10th of solar metallicity.
Low-surface-brightness galaxies have large characteristic radii, large gas fraction and are in general dark matter dominated; they are quite unevolved objects. Their total gas content is similar to that of normal galaxies (McGaugh & de Blok 1997). But CO is not detected in LSB (de Blok & van der Hulst 1998). Due to their low surface density, below the threshold for star formation, these galaxies have a very low efficiency of star formation (Van Zee et al 1997). The cause could be the absence of companions, since LSB live in poor environments (Zaritsky & Lorrimer 1993). It is well known that galaxy interactions, by driving in a high amount of gas, trigger star formation.
2.3.Ultra-luminous IRAS galaxies
At the opposite, there exists a class of galaxies, characterized by their bursts of star formation; these are ultra-luminous in far-infrared, because of the emission of dust heated by the new stars. These objects possess large amounts of gas, particularly condensed in the inner parts, certainly due to interactions and mergers. CO emission is highly enhanced in these starbursting galaxies, and large H2 masses are deduced, even with a modified (lower than standard) conversion ratio (Solomon et al 1997). The prototype of these objects is the nearby Arp220: new CO interferometer data show that CO is in rotating nuclear disks (Downes & Solomon 1998), where the surface density of gas is about 30% of the total surface density.