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