High-redshift star formation in a time-dependent Lyman-Werner background

The first generation of stars produces a background of Lyman-Werner (LW) radiation which can photodissociate molecular hydrogen, increasing the mass of dark matter haloes required to host star formation. Previous studies have determined the critical mass required for efficient molecular cooling with a constant LWbackground. However, the true background is expected to increase rapidly at early times. Neglecting this evolution could underestimate star formation in small haloes that may have started to cool in the past when the LW intensity was much lower. Background evolution is a large source of uncertainty in pre-reionization predictions of the cosmological 21cm signal, which can be observed with future radio telescopes. To address this, we perform zero-dimensional one-zone calculations that follow the density, chemical abundances, and temperature of gas in the central regions of dark matter haloes, including hierarchical growth and an evolving LW background. We begin by studying the physics of haloes subjected to a background that increases exponentially with redshift. We find that when the intensity increases more slowly than J(LW)(z)alpha 10(-z/5), cooling in the past is a relatively small effect. We then self-consistently compute the cosmological LW background over z = 15-50 and find that cooling in the past due to an evolving background has a modest impact. Finally, we compare these results to three-dimensional hydrodynamical cosmological simulations with varying LW histories. While only a small number of haloes were simulated, the results are consistent with our one-zone calculations.