The brief era of direct collapse black hole formation

Abstract

It has been proposed that the first, intermediate-mass (≈105-6M⊙) black holes might form through direct collapse of unpolluted gas in atomic-cooling haloes exposed to a strong Lyman-Werner (LW) or near-infrared (NIR) radiation. As these systems are expected to be Compton thick, photons above 13.6 eV are largely absorbed and reprocessed into lower energy bands. It follows that direct collapse black holes (DCBHs) are very bright in the LW/NIR bands, typically outshining small high-redshift galaxies by more than 10 times. Once the first DCBHs form, they then trigger a runaway process of further DCBH formation, producing a sudden rise in their cosmic mass density. The universe enters the 'DCBH era' at z ≈ 20 when a large fraction of atomic-cooling haloes are experiencing DCBH formation. By combining the clustering properties of the radiation sources with Monte Carlo simulations, we show that in this scenario the DCBH mass density rises from ~5M⊙ Mpc-3 at z ~ 30 to the peak value ~5 × 105M⊙ Mpc-3 at z ~ 14 in our fiducial model. However, the abundance of active (accreting) DCBHs drops after z ~14, as gas in the potential formation sites (unpolluted haloes with virial temperature slightly above 104 K) is photoevaporated. This effect almost completely suppresses DCBH formation after z ~ 13. The DCBH formation era lasts only ≈ 150 Myr, but it might crucially provide the seeds of the supermassive black holes powering z ~ 6 quasars. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.