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

TL;DR

This work addresses how elevated X-ray backgrounds in the early universe affect direct-collapse black hole seed formation in highly clustered, overdense regions. It extends a semi-analytic, merger-tree–based framework to include X-ray ionization/heating and HD chemistry, exploring LW/X-ray coupling and baryonic streaming motions across a range of $J_{\mathrm{X},21}/J_{21}$ values. The authors find that X-rays can suppress the direct-collapse channel, but in the presence of baryonic streaming and strong clustering, seeds with $M_{\mathrm{BH}} \gtrsim 10^{4}\,M_\odot$ still form, yielding comoving densities of about $\sim 10^{-4}\ \mathrm{Mpc}^{-3}$ and seed $M_{\mathrm{BH}}/M_* \sim 0.01$–0.1, potentially explaining JWST-observed overmassive black holes at $z \sim 3$–6. This links early BH seed formation to the observed high-redshift SMBH population, highlighting the role of X-ray feedback and environmental conditions in shaping the initial BH mass function and subsequent growth.

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.

Figures (7)

• Figure 1: Gas thermal evolution in the main progenitor of a DM halo that grows to $10^{12}~M_\odot$ at $z=6$, computed for $v_\mathrm{bsm}=0$ (left panel) and $1\sigma_\mathrm{bsm}$ (right panel). The line colors represent different X-ray intensities: $J_\mathrm{X,21}/J_{21} = 0$ (purple), $10^{-6}$ (blue), $10^{-5}$ (green), and $10^{-4}$ (dark orange). Colored arrows indicate the moments when $J_{21}$ reaches $10^3$ along each corresponding track. In the case of $v_\mathrm{bsm} = 0$ and $J_\mathrm{X,21}/J_{21} = 10^{-4}$, the gas collapses before $J_{21}$ reaches $10^3$ due to X-ray-induced H$_2$ formation and cooling, and thus no arrow is shown. The resulting BH seed mass for each track is labeled in the corresponding color directly in the figure. This figure demonstrates how X-ray irradiation and baryonic streaming motion affect gas thermal evolution and BH seed formation. In particular, strong X-ray backgrounds can trigger early H$_2$ formation, altering the collapse pathway, while baryonic streaming delays collapse and suppresses the H$_2$-cooling track.
• Figure 2: The number of DM halos that follow each evolutionary track under different X-ray intensities. All halos evolve into $M_\mathrm{h} = 10^{12}~M_\odot$ at $z = 6$. The left and right panels correspond to the cases with $v_\mathrm{bsm} = 0$ and $1\sigma_\mathrm{bsm}$, respectively. The blue, orange, and green bars indicate the (i) H$_2$ track, (ii) H–H$_2$ track, and (iii) H–H track, respectively. The numbers above the bars represent the number of halos in each category. This figure shows how X-ray intensity and baryonic streaming motion influence the thermal evolution pathway of collapsing gas, and thus the likelihood of direct collapse BH formation.
• Figure 3: Mass distribution of seed black holes formed within DM halos that grow to $10^{12}~M_\odot$ at $z=6$. The contributions from different evolutionary tracks are stacked within each mass bin. The upper and lower panels show the results for $v_\mathrm{bsm} = 0$ and $1\sigma_\mathrm{bsm}$, respectively. From left to right, the X-ray intensity increases: $J_\mathrm{X,21}/J_{21} = 0$, $10^{-6}$, $10^{-5}$, and $10^{-4}$. This figure illustrates how baryonic streaming motion and X-ray irradiation affect the mass spectrum of seed black holes by modifying the thermal evolution pathway of the collapsing gas.
• Figure 4: Redshift distribution of seed BH formation within DM halos that grow to $10^{12}~M_\odot$ at $z=6$. The panel layout and color scheme are the same as in Figure \ref{['fig:MassDistribution']}, but the horizontal axis denotes the formation redshift. This figure illustrates how baryonic streaming motion and X-ray irradiation affect the formation redshift by modifying the graviational stability of the gas within DM halos.
• Figure 5: Seed BH mass functions (BHMFs) at $z > 15$ for different parameter combinations. The top and bottom rows correspond to the cases with $v_\mathrm{bsm} = 0$ and $1\sigma_\mathrm{bsm}$, respectively. The left and right columns represent the cases with $J_\mathrm{X,21}/J_\mathrm{21} = 0$ and $10^{-4}$, respectively. The combined BHMFs for $v_\mathrm{bsm} = 0$ and $1\sigma_\mathrm{bsm}$ are shown in Figure 5. This figure highlights how baryonic streaming motion and X-ray intensity independently and jointly influence the shape and amplitude of the seed BHMF at high redshift.