The brief era of direct collapse black hole formation

Abstract

It has been proposed that the first, intermediate-mass ($\\approx\n10^{5-6}~M_\\odot$) black holes might form through direct collapse of unpolluted\ngas in atomic-cooling halos exposed to a strong Lyman-Werner (LW) or\nnear-infrared (NIR) radiation. As these systems are expected to be\nCompton-thick, photons above 13.6 eV are largely absorbed and re-processed into\nlower energy bands. It follows that direct collapse black holes (DCBHs) are\nvery bright in the LW/NIR bands, typically outshining small high-redshift\ngalaxies by more than 10 times. Once the first DCBHs form, they then trigger a\nrunaway process of further DCBH formation, producing a sudden rise in their\ncosmic mass density. The universe enters the "DCBH era" at $z \\approx 20$ when\na large fraction of atomic-cooling halos are experiencing DCBH formation. By\ncombining the clustering properties of the radiation sources with Monte Carlo\nsimulations we show that in this scenario the DCBH mass density rises from\n$\\sim 5$~$M_\\odot$ Mpc$^{-3}$ at $z\\sim 30$ to the peak value $\\sim5\\times10^5\nM_\\odot$ Mpc$^{-3}$ at $z \\sim 14$ in our fiducial model. However, the\nabundance of \\textit{active} (accreting) DCBHs drops after $z \\sim 14$, as gas\nin the potential formation sites (unpolluted halos with virial temperature\nslightly above $10^4$~K) is photoevaporated. This effect almost completely\nsuppresses DCBH formation after $z\\sim 13$. The DCBH formation era lasts only\n$\\approx 150$ Myr, but it might crucially provide the seeds of the supermassive\nblack holes (SMBHs) powering $z\\sim6$ quasars.\n