We use the FIRE-2 cosmological simulations to study the formation of a virial temperature, quasi-static 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 ~ 1012 M_sun, the cooling time of shocked gas in the inner CGM (~ 0.1 Rvir, where Rvir is the virial radius) exceeds the local free-fall time. The inner CGM then experiences a transition from on average sub-virial temperatures (T « Tvir), large pressure fluctuations and supersonic inflow/outflow velocities, to virial temperatures (T ~ Tvir), uniform pressures and subsonic velocities. This transition occurs when the outer CGM (~ 0.5 Rvir) is already subsonic and has a temperature ~ Tvir, 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 virialization of the inner CGM coincides with abrupt changes in the properties of 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 hence 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.
Stern, Jonathan, Faucher-Giguère, Claude-André, Fielding, Drummond, Quataert, Eliot, Hafen, Zachary, Gurvich, Alexander B., Ma, Xiangcheng, Byrne, Lindsey, El-Badry, Kareem, Anglés-Alcázar, Daniel, Chan, T. K., Feldmann, Robert, Kereš, Dušan, Wetzel, Andrew, Murray, Norman, Hopkins, Philip F.. Virialization of the inner CGM in the FIRE simulations and implications for galaxy discs, star formation and feedback. ApJ, in press, 2021