The suppression of direct collapse black hole formation by soft X-ray irradiation

Abstract

The origin of supermassive black holes (SMBHs) in galactic nuclei is one of the major unsolved problems in astrophysics. One hypothesis is that they grew from >10^5 M_sun black holes that formed in the `direct collapse' of massive gas clouds that have low concentrations of both metals and molecular hydrogen (H_2). Such clouds could form in the early (z>10) Universe if pre-galactic gas is irradiated by H_2-photodissociating, far-ultraviolet (FUV) light from a nearby star-forming galaxy. In this work, we re-examine the critical FUV flux J_crit that is required to keep H_2 photodissociated and lead to direct collapse. We submit that the same galaxies that putatively supply the extraordinary FUV fluxes required for direct collapse should also produce copious amounts of soft X-rays, which work to offset H_2 photodissociation by increasing the ionization fraction and promoting H_2 formation. Accounting for this effect increases the value of J_crit by a factor of at least 3-10, depending on the brightness temperature of FUV radiation. This enhancement of J_crit suppresses the abundance of potential direct collapse sites at z>10 by several orders of magnitude. Recent studies---without accounting for the soft X-rays from the FUV source galaxies---had already arrived at large values of J_crit that implied that direct collapse may occur too rarely to account for the observed abundance of high-redshift quasars. Our results suggest that J_crit should be even higher than previously estimated, and pose an additional challenge for the direct collapse scenario via strong FUV radiation to explain the high-redshift quasar population.