On the interstellar medium and star formation demographics of galaxies in the local universe

We present a demographic analysis of integrated star formation and gas properties for a sample of galaxies representative of the overall population at z similar to 0. This research was undertaken in order to characterize the nature of star formation and interstellar medium (ISM) behaviour in the local Universe, and test the extent to which global star formation rates (SFRs) can be seen as dependent on the interstellar gas content. Archival 21-cm derived H i data are compiled from the literature, and are combined with CO (J = 1 -> 0) derived H(2) masses to calculate and characterize the total gas content for a large sample of local galaxies. The distribution in stellar mass-normalized H i content is found to exhibit the noted characteristic transition at stellar masses of similar to 3 x 1010 M(circle dot), turning off towards low values, but no such transition is observed in the equivalent distribution of molecular gas. H alpha based SFRs and specific star formation rates (SSFRs) are also compiled for a large (1110) sample of local galaxies. We confirm two transitions as found in previous work: a turnover towards low SFRs at high luminosities, indicative of the quenching of SF characteristic of the red sequence; and a broadening of the SF distribution in low-luminosity dwarf galaxies, again to extremely low SFRs of < 10-3 M(circle dot) yr-1. However, a new finding is that while the upper luminosity transition is mirrored by the turnover in H i content, suggesting that the low SFRs of the red sequence result from a lack of available gas supply, the transition towards a large spread of SFRs in the least luminous dwarf galaxies is not matched by a prominent increase in scatter in gas content. Possible mass-dependent quenching mechanisms are discussed, along with speculations that in low-mass galaxies, the H alpha luminosity may not faithfully trace the SFR.