The formation of hot gaseous haloes around galaxies

One of the major goals of modern galaxy formation theory is to understand the physical mechanisms that halt the star formation process, by either removing, heating or preventing the infall of cold gas onto the galactic disc. X-ray observations suggest that for haloes hosting massive galaxies the majority of baryonic matter resides not in the galaxies, but in the halo in the form of virialized hot gas (e.g. Lin et al. 2003; Crain et al. 2010; Anderson & Bregman 2011). This work (Correa, C. A.; Schaye, J.; Wyithe, J. S. B.; Duffy, A. R.; Theuns, T.; Crain, R. A.; Bower, R. G., MNRAS, 2018, 473, 1) investigates the formation of the hot gaseous corona (also refereed to as ‘hot halo’ or ‘hot atmosphere’) around galaxies, that may help reduce the rate of infall of gas onto galaxies, and has been suggested to explain the observed galaxy bimodality (Dekel & Birnboim 2006).

The hot gaseous corona is produced as a result of an important heating process, that was initially discussed by Rees & Ostriker (1977), Silk (1977), Binney (1977) and White & Rees (1978), and later in the context of the cold dark matter paradigm by e.g. White & Frenk (1991), in an attempt to explain the reduced efficiency of star formation within massive haloes. They proposed that while a dark matter halo relaxes to virial equilibrium, gas falling into it experiences a shock, and determined the cooling time of gas behind the shock. As long as the cooling time is shorter than the dynamical time, the infalling gas cools (inside the current ‘cooling radius’) and settles onto the galaxy. If, on the other hand, the cooling time exceeds the dynamical time, the gas is not able to radiate away the thermal energy that supports it. It therefore adjusts its density and temperature quasi statically, forming a hot hydrostatic halo atmosphere, pressure supported against gravitational collapse. Over the past decade, the works of Birnboim & Dekel (2003) and Dekel & Birnboim (2006, hereafter DB06) investigated the stability of accretion shocks around galaxies, and concluded that a hot atmosphere forms when the compression time of shocked gas is larger than its cooling time, occurring when haloes reach a mass of about 10^{11.7}M_{\odot}.

In this work we use a suite of hydrodynamical cosmological simulations from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) project to investigate the formation of hot hydrostatic haloes and their dependence on feedback mechanisms. We find that the appearance of a strong bimodality in the probability density function (PDF) of the ratio of the radiative cooling and dynamical times for halo gas provides a clear signature of the formation of a hot corona. Haloes of total mass 10^{11.5}-10^{12}M_{\odot} develop a hot corona independent of redshift, at least in the interval z = 0-4 where the simulation has sufficiently good statistics. We analyse the build up of the hot gas mass in the halo, M_{hot}, as a function of halo mass and redshift and find that while more energetic galactic winds powered by SNe increases M_{hot}, AGN feedback reduces it by ejecting gas from the halo. We also study the thermal properties of gas accreting onto haloes and measure the fraction of shock-heated gas as a function of redshift and halo mass. We develop analytic and semianalytic approaches to estimate a ‘critical halo mass’, M_{crit}, for hot halo formation. We find that the mass for which the heating rate produced by accretion shocks equals the radiative cooling rate, reproduces the mass above which haloes develop a significant hot atmosphere. This yields a mass estimate of M_{crit}=10^{11.7}M_{\odot} at z=0, which agrees with the simulation results. The value of M_{crit} depends more strongly on the cooling rate than on any of the feedback parameters.