Over-cooled haloes at z > 10: a route to form low-mass first stars

TL;DR

This study investigates how mergers in high-redshift haloes can trigger HD cooling, enabling the formation of low-mass stars in primordial environments, by combining previous shock studies with dark matter simulations.

Contribution

It demonstrates that a significant fraction of high-redshift haloes can undergo HD cooling due to mergers, expanding understanding of early star formation pathways.

Findings

Fraction of over-cooled haloes can reach ~0.6 at z~10.

Over-cooled haloes are common below the atomic cooling limit.

High-redshift mini-haloes can cool to the CMB temperature, enabling low-mass star formation.

Abstract

It has been shown by Shchekinov & Vasiliev2006 (SV06) that HD molecules can be an important cooling agent in high redshift z >10 haloes if they undergo mergers under specific conditions so suitable shocks are created. Here we build upon Prieto et al. (2012) who studied in detail the merger-generated shocks, and show that the conditions for HD cooling can be studied by combining these results with a suite of dark-matter only simulations. We have performed a number of dark matter only simulations from cosmological initial conditions inside boxes with sizes from 1 to 4 Mpc. We look for haloes with at least two progenitors of which at least one has mass M > M_cr (z), where M_cr (z) is the SV06 critical mass for HD over-cooling. We find that the fraction of over-cooled haloes with mass between M_cr (z) and 10^{0.2} M_cr (z), roughly below the atomic cooling limit, can be as high as ~ 0.6 at z ~ 10 depending on the merger mass ratio. This fraction decreases at higher redshift reaching a value ~0.2 at z ~ 15. For higher masses, i.e. above 10^{0.2} M_cr (z) up to 10^{0.6} M_cr (z), above the atomic cooling limit, this fraction rises to values ~ 0.8 until z ~ 12.5. As a consequence, a non negligible fraction of high redshift z > 10 mini-haloes can drop their gas temperature to the Cosmic Microwave Background temperature limit allowing the formation of low mass stars in primordial environments.