Virialization of high redshift dark matter haloes

TL;DR

This study investigates the virial state of high redshift dark matter haloes, revealing they are generally not virialized and possess excess kinetic energy, which influences early galaxy formation processes.

Contribution

It provides the first detailed analysis of the virialization status of high redshift dark matter haloes, including the effects of environment and halo properties.

Findings

Most haloes are not virialized at high redshift.

Including surface pressure reduces but does not eliminate excess kinetic energy.

Halo proximity and spin correlate with deviations from virial equilibrium.

Abstract

We present results of a study of the virial state of high redshift dark matter haloes in an N-body simulation. We find that the majority of collapsed, bound haloes are not virialized at any redshift slice in our study ($z=15-6$) and have excess kinetic energy. At these redshifts, merging is still rampant and the haloes cannot strictly be treated as isolated systems. To assess if this excess kinetic energy arises from the environment, we include the surface pressure term in the virial equation explicitly and relax the assumption that the density at the halo boundary is zero. Upon inclusion of the surface term, we find that the haloes are much closer to virialization, however, they still have some excess kinetic energy. We report trends of the virial ratio including the extra surface term with three key halo properties: spin, environment, and concentration. We find that haloes with closer neighbors depart more from virialization, and that haloes with larger spin parameters do as well. We conclude that except at the lowest masses ($M < 10^6 \Msun$), dark matter haloes at high redshift are not fully virialized. This finding has interesting implications for galaxy formation at these high redshifts, as the excess kinetic energy will impact the subsequent collapse of baryons and the formation of the first disks and/or baryonic structures.