Kavli Affiliate: Kohei Inayoshi
| First 5 Authors: Kazutaka Kimura, Kohei Inayoshi, Kazuyuki Omukai, ,
| Summary:
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
$zsimeq 15$, 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.
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