The Fate of the Interstellar Medium in Early-type Galaxies. III. The Mechanism of Interstellar Medium Removal and the Quenching of Star Formation

Understanding how galaxies quench their star formation is crucial for studies of galaxy evolution. Quenching is related to a decrease of cold gas. In the first paper we showed that the dust removal timescale in early-type galaxies (ETGs) is about 2.5 Gyr. Here we present carbon monoxide and 21 cm hydrogen line observations of these galaxies and measure the timescale of removal of the cold interstellar medium (ISM). We find that all the cold ISM components (dust and molecular and atomic gas) decline at similar rates. This allows us to rule out a wide range of potential ISM-removal mechanisms (including starburst-driven outflows, astration, or a decline in the number of asymptotic giant branch stars), and artificial effects like the stellar mass-age correlation, environmental influence, mergers, and selection bias, leaving ionization by evolved low-mass stars and ionization/outflows by Type Ia supernovae or active galactic nuclei as viable mechanisms. We also provide evidence for an internal origin of the detected ISMs. Moreover, we find that the quenching of star formation in these galaxies cannot be explained by a reduction in the gas amount alone, because the star formation rates (SFRs) decrease faster (on a timescale of about 1.8 Gyr) than the amount of cold gas. Furthermore, the star formation efficiency (SFE) of the ETGs ( SFE equivalent to SFR/MH2 ) is lower than that of star-forming galaxies, whereas their gas mass fractions ( fH2 equivalent to MH2/M* ) are normal. This may be explained by the stabilization of gas against fragmentation, for example due to morphological quenching, turbulence, or magnetic fields.