Enhanced direct collapse due to Lyman α feedback

¹ X Theoretical Design and the Center for Theoretical Astrophysics, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
e-mail: jlj@lanl.gov
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway

Received: 4 November 2016
Accepted: 26 February 2017

Abstract

We assess the impact of trapped Lyman α cooling radiation on the formation of direct collapse black holes (DCBHs). We apply a one-zone chemical and thermal evolution model, accounting for the photodetachment of H⁻ ions, precursors to the key coolant H₂, by Lyman α photons produced during the collapse of a cloud of primordial gas in an atomic cooling halo at high redshift. We find that photodetachment of H⁻ by trapped Lyman α photons may lower the level of the H₂-dissociating background radiation field required for DCBH formation substantially, dropping the critical flux by up to a factor of a few. This translates into a potentially large increase in the expected number density of DCBHs in the early Universe, and supports the view that DCBHs may be the seeds for the BHs residing in the centers of a significant fraction of galaxies today. We find that detachment of H⁻ by Lyman α has the strongest impact on the critical flux for the relatively high background radiation temperatures expected to characterize the emission from young, hot stars in the early Universe. This lends support to the DCBH origin of the highest redshift quasars.