Star Formation in Simulated Galaxies: Understanding the Transition to Quiescence at $3\times10^{10}$ M$_\odot$

Abstract: Star formation in galaxies relies on the availability of cold, dense gas, which, in turn, relies on factors internal and external to the galaxies. In order to provide a simple model for how star formation is regulated by various physical processes in galaxies, we analyse data at redshift $z=0$ from a hydrodynamical cosmological simulation that includes prescriptions for star formation and stellar evolution, active galactic nuclei (AGN), and their associated feedback processes. This model can determine the star formation rate (SFR) as a function of galaxy stellar mass, gas mass, black hole mass, and environment. We find that gas mass is the most important quantity controlling star formation in low-mass galaxies, and star-forming galaxies in dense environments have higher SFR than their counterparts in the field. In high-mass galaxies, we find that black holes more massive than $\sim10^{7.5}$ M$\odot$ can be triggered to quench star formation in their host; this mass scale is emergent in our simulations. Furthermore, this black hole mass corresponds to a galaxy bulge mass $\sim2\times10^{10}$ M$\odot$, consistent with the mass at which galaxies start to become dominated by early types ($\sim3\times10^{10}$ M$_\odot$, as previously shown in observations by Kauffmann et al.). Finally, we demonstrate that our model can reproduce well the SFR measured from observations of galaxies in the GAMA and ALFALFA surveys.