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

Kazutaka Kimura, Kohei Inayoshi, Kazuyuki Omukai

First listed 2025-04-14 | Last updated 2025-10-20

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.

Short digest

Semi-analytic merger-tree modeling with observationally calibrated X-ray irradiation (guided by HERA’s 21 cm non-detections) shows that strong X-ray backgrounds boost H2 formation, steering most overdense halos off the classic direct-collapse path. Including baryonic streaming motion preserves an atomic-cooling route in a subset of cases, still yielding massive seeds. The model predicts seeds with M_BH ≳ 10^4 Msun by z ≃ 15 and an abundance ≈10^-4 Mpc^-3, sufficient to account for SMBHs verified by JWST spectroscopy at 3<z<6. X-ray ionization prevents formation of extremely overmassive BHs comparable to their hosts, while establishing BH-to-stellar mass ratios ≃0.01–0.1 at seeding.

Key figures to inspect

• Figure 1: Follow the temperature–density tracks across J_X and v_bc to see when gas flips from atomic to H2-cooling; compare the annotated seed masses to quantify how strong X-rays trigger early H2 formation while streaming delays collapse and preserves the atomic track.
• Figure 2: Read the halo counts in the H2, H–H2, and H–H channels versus J_X and v_bc to measure how X-rays shift systems away from direct collapse, and how streaming partially restores the atomic-cooling pathways.
• Figure 3: Inspect the stacked seed-mass distributions by track for each J_X and v_bc; identify where M_BH ≳ 10^4 Msun seeds arise and how their incidence declines as J_X increases, with streaming pushing the distribution to higher masses.
• Figure 4: Compare seed BH mass functions at the seeding epoch across the J_X–v_bc grid; check whether the amplitude approaches ≈10^-4 Mpc^-3 and how suppressing atomic-cooling tracks reshapes the BHMF.