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Virialization of the inner CGM in the FIRE simulations and implications for galaxy disks, star formation, and feedback

  • Author(s): Stern, J;
  • Faucher-Giguère, CA;
  • Fielding, D;
  • Quataert, E;
  • Hafen, Z;
  • Gurvich, AB;
  • Ma, X;
  • Byrne, L;
  • El-Badry, K;
  • Anglés-Alcázar, D;
  • Chan, TK;
  • Feldmann, R;
  • Kereš, D;
  • Wetzel, A;
  • Murray, N;
  • Hopkins, PF
  • et al.
Abstract

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 ∼1012 Me, the cooling time of shocked gas in the inner CGM (∼0.1Rvir, where Rvir is the virial radius) exceeds the local free-fall time. The inner CGM then experiences a transition from on average subvirial 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 (∼0.5Rvir) 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 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.

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