1995A&A...300L..13B

We use CO observations of spiral galaxies in the Coma/A1367 supercluster to study the relation between their H2 mass and their star formation rate. The galaxies form a complete optically-selected sample, which is dominated by luminous objects. Contrary to the results of several previous studies, we find for this sample a positive correlation between the molecular gas content as derived from the ¹²CO(1-O) line intensity using a standard conversion factor X, and the star formation activity as traced by the Half equivalent width and by the far-infrared emission, all quantities being normalized to the H luminosity or to the disk area of the galaxy. No such correlation had been found previously for samples of closer galaxies which are dominated by lower-luminosity objects. However for Virgo cluster galaxies this correlation exists for the brightest objects and is only absent at lower luminosities. Rather than indicating a genuine lack of relation between molecular gas and star formation, the observations may indicate that the use of a standard CO line intensity/H2 mass conversion factor X is meaningful only in massive galaxies such as the Milky Way. In low-mass galaxies the higher far-UV radiation field and the lower dust-to-gas ratio increase the amount of photodissociation and raise the numerical value of the conversion factor. Consequently, the amount of molecular hydrogen is underestimated for these galaxies if the conventional value of X is used. Presumably the value of X varies from galaxy to galaxy especially for the low-luminosity ones due to the strong non-linearity of the processes involved, introducing a large dispersion in the relation between star formation and CO emission. J%K galaxies: general; ISM - star formation - radio lines: galaxies