  Observations indicate that galaxies with high stellar masses or in dense
environments have low specific star formation rates, i.e. they are quenched.
Based on cosmological hydrodynamic simulations that include a prescription
where quenching occurs in regions dominated by hot (>10^5 K) gas, we argue that
this hot gas quenching in halos >10^12 Msun drives both mass quenching (i.e.
central quenching) and environment quenching (i.e. satellite quenching). These
simulations reproduce a broad range of locally observed trends among quenching,
halo mass, stellar mass, environment, and distance to halo center. We show that
mass quenching is independent of environment because 10^12-10^13 Msun
"quenching halos" -- those where most mass quenching occurs -- inhabit a large
range of environments. On the other hand, environment quenching is independent
of stellar mass because galaxies of all stellar masses may live in dense
environments as satellites of groups and clusters. Furthermore, satellite
galaxies show signs of mass quenching independent of halo mass because massive
satellites at z=0 have typically been mass quenched as centrals in their own
hot halos at higher z -- a kind of pre-processing. As in observations, the
fraction of quenched satellites increases with halo mass and decreases with
distance to the center of the group or cluster. We investigate quenched
centrals in low-mass halos (<10^12 Msun), and show that most of these are
ejected former satellites of groups or clusters, while about 20 per cent were
never satellites but are enveloped in hot gas that extends up to 3 Rvir from
the centers of clusters. The agreement of our model with key observational
trends suggests that hot gas in massive halos plays a leading role in quenching
low-redshift galaxies.
