Virialization of the inner CGM in the FIRE simulations and implications
  for galaxy discs, star formation and feedback
  We use the FIRE-2 cosmological simulations to study the formation of a
quasi-static, virial-temperature gas phase in the circumgalactic medium (CGM)
at redshifts 0<z<5, and how the formation of this virialized phase affects the
evolution of galactic discs. We demonstrate that when the halo mass crosses
~10^12 M_sun, the cooling time of shocked gas in the inner CGM (~0.1 R_vir,
where R_vir is the virial radius) exceeds the local free-fall time. The inner
CGM then experiences a transition from on average sub-virial temperatures
(T<<T_vir), large pressure fluctuations and supersonic inflow/outflow
velocities, to virial temperatures (T~T_vir), uniform pressures and subsonic
velocities. This transition occurs when the outer CGM (~0.5 R_vir) is already
subsonic and has a temperature ~T_vir, indicating that the longer cooling times
at large radii allow the outer CGM to virialize at lower halo masses than the
inner CGM. This outside-in CGM virialization scenario is in contrast with
inside-out scenarios commonly envisioned based on more idealized simulations.
We demonstrate that inner CGM virialization coincides with abrupt changes in
the central galaxy and its stellar feedback: the galaxy settles into a stable
rotating disc, star formation transitions from `bursty' to `steady,' and
stellar-driven galaxy-scale outflows are suppressed. Our results thus suggest
that CGM virialization is initially associated with the formation of
rotation-dominated thin galactic discs, rather than with the quenching of star
formation as often assumed.
