# Resolving the Formation of Protogalaxies. I. Virialization

## TL;DR

This paper uses advanced simulations to analyze how baryonic matter in early protogalaxies virializes, revealing the roles of shocks, turbulence, and cooling in galaxy formation.

## Contribution

It provides detailed insights into the virialization process of baryons in early protogalaxies, including the effects of cooling and turbulence, using high-resolution cosmological simulations.

## Key findings

- Virial shocks occur near the virial radius in non-cooling halos.
- Cooling causes the virial shock to shrink and filaments to penetrate deeper.
- Turbulence in cooling halos is supersonic and driven by mergers.

## Abstract

(Abridged) 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 10^4 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 also 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 two 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 one to three 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.