Suppression of H_2 Cooling in the Ultraviolet Background

TL;DR

This paper demonstrates through simulations that ultraviolet radiation suppresses molecular hydrogen cooling in early halos, significantly impacting models of early universe structure formation.

Contribution

It provides the first detailed simulation-based analysis showing UV background suppresses H_2 cooling, altering predictions of early star formation.

Findings

Cooling halos are much smaller than previously thought.

The abundance of star-forming halos depends exponentially on the critical mass.

Models underestimate early star formation by orders of magnitude.

Abstract

The first luminous objects in the concordance cosmology form by molecular hydrogen cooling in dark matter dominated halos of masses ~10^6 M_sun. We use Eulerian adaptive mesh refinement simulations to demonstrate that in the presence of a large soft ultraviolet radiation background, molecular hydrogen is the dominant coolant. Even for very large radiation backgrounds, the halo masses that cool and collapse are up to two orders of magnitude smaller than the halos that cool via atomic hydrogen line cooling. The abundance of cooling halos and the cosmic mass fraction contained within them depends exponentially on this critical mass scale. Consequently, the majority of current models of cosmological reionization, chemical evolution, supermassive black hole formation, and galaxy formation underestimate the number of star forming progenitors of a given system by orders of magnitude. At the highest redshifts, this disagreement is largest. We also show that even in the absence of residual electrons, collisional ionization in central shocks create a sufficient amount of electrons to form molecular hydrogen and cool the gas in halos of virial temperatures far below the atomic cooling limit.