Overcooled haloes at z ≥10: A route to form low-mass first stars

Abstract

It has been shown by Shchekinov and Vasiliev (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. 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 > Mcr(z), where Mcr(z) is the SV06 critical mass for HD overcooling.We find that the fraction of overcooled haloes with mass between Mcr(z) and 100.2Mcr(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 100.2Mcr(z) up to 100.6Mcr(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. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.