Resolving the Formation of Protogalaxies. I. Virialization

Galaxies form in hierarchically assembling dark matter halos. With cosmological three-dimensional adaptive mesh refinement simulations, we explore in detail the virialization of baryons in the concordance cosmology, including optically thin primordial gas cooling. We focus on early protogalaxies with virial temperatures of 10⁴ K and their progenitors. Without cooling, virial heating occurs in shocks close to the virial radius for material falling in from voids. Material in dense filaments penetrates deeper to about half that radius. With cooling, the virial shock position shrinks, and the filaments reach scales as small as a third the virial radius. The temperatures in protogalaxies found in adiabatic simulations decrease by a factor of 2 from the center and show flat entropy cores. In cooling halos, the gas reaches virial equilibrium with the dark matter potential through its turbulent velocities. We observe turbulent Mach numbers ranging from 1 to 3 in the cooling cases. This turbulence is driven by the large scale merging and, interestingly, remains supersonic in the centers of these early galaxies, even in the absence of any feedback processes. The virial theorem is shown to approximately hold over 3 orders of magnitude in length scale, with the turbulent pressure prevailing over the thermal energy. The turbulent velocity distributions are Maxwellian and by far dominate the small rotation velocities associated with the total angular momentum of the galaxies. Decomposing the velocity field using the Cauchy-Stokes theorem, we show that ample amounts of vorticity are present around shocks even at the very centers of these objects. In the cold flow regime of galaxy formation for halo masses below 10¹² M_☉, this dominant role of virialization-driven turbulence should play an important role in star formation, as well as the build up of early magnetic fields.