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

Kohei Inayoshi, Takamitsu L. Tanaka

Published 2014-11-10Version 1

The origins 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 Msun 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. The key uncertainties with this scenario are (1) how strong the FUV flux must be to sufficiently suppress the H2 abundance to prevent fragmentation and ordinary star formation; and (2) whether the requisite conditions arise frequently enough in nature to account for the observed number density of SMBHs (luminous quasars) at high redshifts. In this work, we re-examine the critical FUV flux J_crit that is required to keep H2 photodissociated and lead to direct collapse. We show that J_crit could be much higher than previously believed if the same FUV sources also produce X-rays, which can work to offset H2 photodissociation by increasing the ionization fraction and promoting H2 formation via electron-catalyzed reactions. We stress that soft (~ 1 keV) X-rays are far more effective at promoting H2 formation than hard (~ 10 keV) X-rays. Further, we estimate how much soft X-rays can suppress the number density of direct-collapse black holes compared to previous calculations. We find that, even for conservative sets of assumptions, if J_crit is higher than 400-1000 then direct collapse would occur too rarely to explain the observed abundance of z>6 quasars.