Low-temperature primordial gas in merging halos

TL;DR

This paper investigates how shock waves in merging dark matter halos can cool baryonic gas to near the CMB temperature, potentially leading to the formation of extremely metal-poor low-mass stars in the early universe.

Contribution

It provides estimates of the fraction of primordial gas cooled by HD lines in merging halos and explores its implications for early star formation.

Findings

The cooled gas fraction increases with halo mass, reaching up to 0.3 for very massive halos.

Shock-induced cooling can contribute to early star formation, including the formation of metal-poor stars.

Primordial gas cooling in shocks may explain the origin of some extremely metal-poor stars in the Milky Way.

Abstract

Thermal regime of the baryons behind shock waves arising in the process of virialization of dark matter halos is governed at cetrain conditions by radiation of HD lines. A small fraction of the shocked gas can cool down to the temperature of the cosmic microwave background (CMB). We estimate an upper limit for this fraction: at $z=10$ it increases sharply from about $q_{_T}\sim 10^{-3}$ for dark halos of $M=5\times 10^7\msun$ to $\sim 0.1$ for halos with $M=10^8\msun$. Further increase of the halo mass does not lead however to a significant growth of $q_T$ -- the asymptotic value for $M\gg 10^8\msun$ is of 0.3. We estimate star formation rate associated with such shock waves, and show that they can provide a small but not negligible fraction of the star formation. We argue that extremely metal-poor low-mass stars in the Milky Way may have been formed from primordial gas behind such shocks.