New constraints on direct collapse black hole formation in the early Universe

Abstract

Direct collapse black holes (DCBH) have been proposed as a solution to the challenge of assembling supermassive black holes by $z>6$ to explain the bright quasars observed at this epoch. The formation of a DCBH seed with $\rm M_{BH}\sim10^{4-5}\ \rm M_{\odot}$ requires a pristine atomic-cooling halo to be illuminated by an external radiation field that is sufficiently strong to entirely suppress H$_{2}$ cooling in the halo. Many previous studies have attempted to constrain the critical specific intensity that is likely required to suppress H$_{2}$ cooling, denoted as $J_{\rm crit}$. However, these studies have typically assumed that the incident external radiation field can be modeled with a black-body spectrum. Under this assumption, it is possible to derive a {unique} value for $J_{\rm crit}$ that depends only on the temperature of the black-body. In this study we consider a more realistic spectral energy distribution (SED) for the external source of radiation that depends entirely on its star formation history and age. The rate of destruction of the species responsible for suppressing molecular hydrogen cooling depends on the detailed shape of the SED. Therefore the value of $J_{\rm crit}$ is tied to the shape of the incident SED of the neighbouring galaxy. We fit a parametric form to the rates of destruction of H$_2$ and H$^-$ that permit direct collapse. Owing to this, we find that $J_{\rm crit}$ is not a fixed threshold but can lie anywhere in the range $J_{\rm crit} \sim 0.5$--$10^{3}$, depending on the details of the source stellar population.