From Haloes To Galaxies. Iii. The Gas Cycle Of Local Galaxy Populations

In Dou et al., we introduced the fundamental formation relation (FFR), a tight relation between specific star formation rate (sSFR), H-2 star formation efficiency (SFEH2), and the ratio of H-2 to stellar mass. Here, we show that atomic gas H-I does not follow a similar FFR as H-2. The relation between SFEH. and sSFR shows significant scatter and strong systematic dependence on all of the key galaxy properties that we have explored. The dramatic difference between H-I and H-2 indicates that different processes (e.g., quenching by different mechanisms) may have very different effects on H-I in different galaxies and hence produce different SFEHI-sSFR relations, while the SFEH2-sSFR relation remains unaffected. The fact that the SFEH2-sSFR relation is independent of other key galaxy properties, and that sSFR is directly related to the cosmic time and acts as the cosmic clock, make it natural and very simple to study how different galaxy populations (with different properties and undergoing different processes) evolve on the same SFEH2-sSFR similar to t relation. In the gas regulator model (GRM), the evolution of a galaxy on the SFEH2-sSFR(t) relation is uniquely set by a single mass-loading parameter lambda(net,H2). This simplicity allows us to accurately derive the H-2 supply and removal rates of the local galaxy populations with different stellar masses, from star-forming galaxies to the galaxies in the process of being quenched. This combination of FFR and GRM, together with the stellar metallicity requirement, provide a new powerful tool to study galaxy formation and evolution.