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

TL;DR

This study uses cosmological simulations to show that AGN feedback, especially during quasar phases at high redshift, expels low-entropy gas, solving the over-cooling problem in galaxy groups and low-mass clusters.

Contribution

It demonstrates that AGN feedback, particularly during quasar activity at high redshift, is crucial for reproducing observed properties of galaxy groups and clusters.

Findings

AGN feedback is necessary to match observed group properties.

Low-entropy gas is expelled at high redshift during quasar activity.

Ejection of gas at z=2-4 explains the low density of the intragroup medium.

Abstract

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