Abstract
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, $f_CGM$, 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 $f_CGM$ 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 expulsive 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 $f_CGM$ 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 $f_CGM$ 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.
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