Virialization of the Inner CGM in the FIRE Simulations and Implications for Galaxy Disks, 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 disks. We demonstrate that when the halo mass crosses similar to 10(12) M (circle dot), the cooling time of shocked gas in the inner CGM (similar to 0.1R (vir), where R (vir) is the virial radius) exceeds the local free-fall time. The inner CGM then experiences a transition from on average subvirial temperatures (T MUCH LESS-THAN T (vir)), large pressure fluctuations, and supersonic inflow/outflow velocities to virial temperatures (T similar to T (vir)), uniform pressures, and subsonic velocities. This transition occurs when the outer CGM (similar to 0.5R (vir)) is already subsonic and has a temperature similar to 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 disk, 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 disks, rather than with the quenching of star formation as often assumed.

Automated summary

The study using FIRE-2 cosmological simulations shows that the formation of a virialized gas phase in the circumgalactic medium (CGM) is associated with the transition from subvirial temperatures to virial temperatures, with outer CGM virializing at lower halo masses than inner CGM. This outside-in CGM virialization process contrasts with common inside-out scenarios. The results suggest that CGM virialization is linked to the formation of rotation-dominated thin galactic disks, rather than the quenching of star formation as previously assumed.