Why do Galaxies Stop Forming Stars? New Evidence for the Role of AGN-feedback in Driving Galaxy Bimodality

Abstract

One of the most striking features of the population of local galaxies isthat the distributions of several key galaxy properties are highlybimodal (e.g. color and star formation rate). In general, high massgalaxies in dense environments, with bulge-dominated morphologies andpressure supported kinematics are more frequently passive (non-starforming) than lower mass galaxies in low density environments, withdisc-dominated morphologies and rotationally supported kinematics.Understanding which, if any, of these correlations is causally relatedto the quenching of star formation in galaxies remains anactive and hotly debated area of investigation in modernastrophysics.Theoretically, a wealth of physical processes have beenevoked to account for central galaxy quenching, including halo massquenching from virial shocks, feedback from active galactic nuclei (AGN;in either the quasar or radio mode), stabilizing torques from centralmass concentrations, feedback from supernovae, or even magnetic fieldsinteracting with the hot gas halo.I will present strong new statisticalevidence which suggests that the quenched fraction of local centralgalaxies is primarily related to their central kinematics (Bluck et al.2016; 2017 in prep.). I will show that this is broadly consistent withquenching from AGN feedback, through a detailed comparison with asemi-analytic model and a cosmological hydrodynamical simulation.Using asample of over half a million local galaxies from the SDSS DR7, we go onto develop a number of sophisticated techniques, including machinelearning with artificial neural networks, to rank the importance ofgalaxy properties to quenching (Teimoorinia, Bluck Ellison 2016).We find that properties closely correlated with the central supermassiveblack hole are highly favoured statistically to predict whether a galaxywill be star forming or not. Perhaps surprisingly, stellar mass and halomass have no impact on star formation activity in central galaxiesselected at a fixed black hole mass; and environment is totallyuncorrelated to quenching in centrals.I will conclude by assessing whichphysical mechanisms for quenching are viable in light of our newresults.