The quenching and morphological evolution of central galaxies is
  facilitated by the feedback-driven expulsion of circumgalactic gas
  We examine the connection between the properties of the circumgalactic medium
(CGM) and the quenching and morphological evolution of central galaxies in the
EAGLE and IllustrisTNG simulations. The simulations yield very different median
CGM mass fractions, $f_{\rm CGM}$, as a function of halo mass, $M_{200}$, with
low-mass haloes being significantly more gas-rich in IllustrisTNG than in
EAGLE. Nonetheless, in both cases scatter in $f_{\rm CGM}$ at fixed $M_{200}$
is strongly correlated with the specific star formation rate and the kinematic
morphology of central galaxies. The correlations are strongest for $\sim
L^\star$ galaxies, corresponding to the mass scale at which AGN feedback
becomes efficient. This feedback elevates the CGM cooling time, preventing gas
from accreting onto the galaxy to fuel star formation, and thus establishing a
preference for quenched, spheroidal galaxies to be hosted by haloes with low
$f_{\rm CGM}$ for their mass. In both simulations, $f_{\rm CGM}$ correlates
negatively with the host halo's intrinsic concentration, and hence with its
binding energy and formation redshift, primarily because early halo formation
fosters the rapid early growth of the central black hole (BH). This leads to a
lower $f_{\rm CGM}$ at fixed $M_{200}$ in EAGLE because the BH reaches high
accretion rates sooner, whilst in IllustrisTNG it occurs because the central BH
reaches the mass threshold at which AGN feedback is assumed to switch from
thermal to kinetic injection earlier. Despite these differences, there is
consensus from these state-of-the-art simulations that the expulsion of
efficiently-cooling gas from the CGM is a crucial step in the quenching and
morphological evolution of central galaxies.
