2015MNRAS.450.4350I

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 ≳ 10⁵M_☉ black holes that formed in the `direct collapse' of massive gas clouds that have low concentrations of both metals and molecular hydrogen (H2). Such clouds could form in the early (z ≳ 10) Universe if pre-galactic gas is irradiated by H2-photodissociating, far-ultraviolet (FUV) light from a nearby star-forming galaxy. In this work, we re-examine the critical FUV flux Jcrit that is required to keep H2 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 H2 photodissociation by increasing the ionization fraction and promoting H2 formation. Accounting for this effect increases the value of Jcrit by a factor of at least 3-10, depending on the brightness temperature of FUV radiation. This enhancement of Jcrit 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 Jcrit that implied that direct collapse may occur too rarely to account for the observed abundance of high-redshift quasars. Our results suggest that Jcrit 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.