Halos in Dark Ages: formation and chemistry

TL;DR

This paper models the formation and chemistry of dark matter halos during the Dark Ages in a four-component universe, revealing how halo formation influences molecular densities and the impact of dark energy properties.

Contribution

It provides a detailed relativistic hydrodynamics analysis of halo formation, including the effects of dark energy and molecular chemistry during the Dark Ages.

Findings

Number density of halos with $M\sim10^8-10^9 M_{\odot}$ at $z\sim10$ is comparable to galaxy density.

Molecular densities of H2 and HD are significantly higher in halos than in uniform expansion.

Halo temperature history affects molecular ion concentrations, such as HeH$^+$.

Abstract

Formation of halos in the Dark Ages from initial spherical perturbations is analyzed in a four component Universe (dark matter, dark energy, baryonic matter and radiation) in the approximation of relativistic hydrodynamics. Evolution of density and velocity perturbations of each component is obtained by integration of a system of nine differential equations from $z=10^8$ up to virialization, which is described phenomenologically. It is shown that the number density of dark matter halos with masses $M\sim10^8-10^9\,\mathrm{M_{\odot}}$ virialized at $z\sim10$ is close to the number density of galaxies in comoving coordinates. The dynamical dark energy of classical scalar field type does not significantly influence the evolution of the other components, but dark energy with a small value of effective sound speed can affect the final halo state. Simultaneously, the formation/dissociation of the first molecules have been analyzed in the halos which are forming. The results show that number densities of molecules $\rm{H_2}$ and $\rm{HD}$ at the moment of halo virialization are $\sim10^3$ and $\sim400$ times larger, respectively, than on a uniformly expanding background. It is caused by increased density and rates of reactions at quasilinear and nonlinear evolution stages of density and velocity of the baryonic component of halos. It is shown also that the temperature history of the halo is important for calculating the concentration of molecular ions with low binding energy. So, in a halo with virial temperature $\sim10^5$ K the number density of the molecular ion HeH$^+$ is approximately 100 times smaller than that on the cosmological background.