Massive Black Hole Seed Formation in Strong X-ray Environments at High Redshift

Abstract

Direct collapse of pristine gas in early galaxies is a promissing pathway for forming supermassive black holes (BHs) powering active galactic nuclei (AGNs) at the epoch of reionization (EoR). This seeding mechanism requires suppression of molecular hydrogen (H$_2$) cooling during primordial star formation via intense far-ultraviolet radiation from nearby starburst galaxies clustered in overdense regions. However, non-detection of 21 cm signals from the EoR reported by the Hydrogen Epoch of Reionization Array (HERA) experiment suggests that such galaxies may also emit X-rays more efficiently than in the local universe, promoting H$_2$ production and thereby potentially quenching massive BH seed formation. In this study, we examine the thermal and chemical evolution of collapsing gas in dark matter halos using a semi-analytic model incorporating observationally calibrated X-ray intensities. We find that strong X-ray irradiation, as suggested by HERA, significantly suppresses direct collapse and leads most halos to experience H$_2$ cooling. Nevertheless, massive BH seeds with $M_\mathrm{BH} \gtrsim 10^4~M_\odot$ still form by $z\simeq 15$, particularly in regions with baryonic streaming motion, and their abundance reaches $\sim 10^{-4}~\mathrm{Mpc}^{-3}$ sufficient to explain the SMBHs identified by JWST spectroscopy at $3<z<6$. While the formation of highly overmassive BHs with masses comparable to their host galaxies is prohibited by X-ray ionization, our model predicts that BH-to-stellar mass ratios of $\simeq 0.01-0.1$ were already established at seeding.