Gas expulsion by quasar-driven winds as a solution to the overcooling problem in galaxy groups and clusters

Abstract

Galaxy groups are not scaled down versions of massive galaxy clusters - the\nhot gas in groups (known as the intragroup medium, IGrM for short) is, on\naverage, less dense than the intracluster medium, implying that one or more\nnon-gravitational processes (e.g., radiative cooling, star formation, and/or\nfeedback) has had a relatively larger effect on groups. In the present study,\nwe compare a number of cosmological hydrodynamic simulations that form part of\nthe OverWhelmingly Large Simulations project to isolate and quantify the\neffects of cooling and feedback from supernovae (SNe) and active galactic\nnuclei (AGN) on the gas. This is achieved by comparing Lagrangian thermal\nhistories of the gas in the different runs, which were all started from\nidentical initial conditions. While radiative cooling, star formation, and SN\nfeedback are all necessary ingredients, only runs that also include AGN\nfeedback are able to successfully reproduce the optical and X-ray properties of\ngroups and low-mass clusters. We isolate how, when, and exactly what gas is\nheated by AGN. Interestingly, we find that the gas that constitutes the\npresent-day IGrM is that which was not strongly heated by AGN. Instead, the low\nmedian density/high median entropy of the gas in present-day groups is achieved\nby the ejection of lower entropy gas from low-mass progenitor galaxies at high\nredshift (primarily 2 < z < 4). This corresponds to the epoch when supermassive\nblack holes accreted most of their mass, typically at a rate that is close to\nthe Eddington limit (i.e., when the black holes are in a `quasar mode').\n