1997A&A...327..522B

We present ¹²CO(J=1-0) line observations of 73 spiral galaxies mostly in the Coma/A1367 supercluster. From these data, combined with data available in the literature, we extract the first complete, optically selected sample (m_pg<15.2) of 37 isolated and of 27 cluster galaxies. Adopting a standard conversion factor X=N(H₂)/I(CO), we estimate that the molecular hydrogen content of isolated spiral galaxies is, on average, 20% of the atomic hydrogen reservoir, significantly lower than previous estimates based on samples selected by FIR criteria, thus biased towards CO rich objects. We show that the frequency distributions of the CO deficiency parameter, defined as the difference between the expected and the observed molecular gas content of a galaxy of given luminosity (or linear diameter), computed separately for cluster and isolated galaxies, are not significantly different, indicating that the environment does not affect the molecular gas content of spiral discs. A well defined relationship exists between M_i(H₂) and the star formation activity in bright galaxies, while it is weaker at lower luminosities. We interpret this finding as indicating that CO emission traces relatively well the H₂ mass only in high-mass galaxies, such as the Milky Way. On the other hand, in low-mass spirals the higher far-UV radiation field produced by young O-B stars and the lower metallicity cause the photodissociation of the diffuse molecular gas, weakening the expected relationship between star formation and the CO emission. The conversion factor between the CO line intensity and the amount of molecular hydrogen being ill-determined and variable with the UV flux and abundances, it is difficult to assess the relationship between the star formation and the amount of molecular hydrogen.