Formation of over-massive black holes in high-redshift disk galaxies via globular cluster accretion

TL;DR

The paper addresses the puzzle of over-massive black holes in the early universe revealed by JWST. It introduces an analytic GC migration and tidal disruption–driven accretion model in compact high-redshift disks, deriving formation conditions as functions of $M_{ m h}$, $z$, and $\\\lambda$, and predicting $M_{ m BH}$ and a BH mass function via $M_{ m BH} = f_{ m TDE}\,\\varepsilon_{ m SF,min}\,f_d\frac{\\Omega_{ m b}}{\\Omega_{ m m}}\,M_{ m h}$. The results show that at $z=10$ halos with $M_{ m h}\\approx 10^{11}~M_\\odot$ and $\\lambda\\approx 0.02$ can form $M_{ m BH}\\approx 2.3\\times 10^{8}~M_\\odot$, achieving $M_{ m BH}/M_\\star\\sim 0.1$, and that the derived BH mass functions reproduce JWST candidate densities at high redshift while predicting more massive BHs in rarer halos. This GC-driven channel offers a plausible, testable pathway for rapid BH growth in the early universe, highlighting the roles of GC formation, migration, and TDE-fed accretion, with concrete predictions for future JWST and lensing surveys.

Abstract

Recent observations with the James Webb Space Telescope (JWST) have suggested the existence of over-massive black holes (OMBHs) in high-redshift galaxies. In this paper, we propose a new mechanism for the formation of OMBHs, based on the accretion of globular clusters (GCs) in compact disk galaxies. We derive the conditions under which OMBHs can form, focusing on key parameters such as halo mass, redshift, and halo spin parameter. Our results show that at redshift $z = 10$, a halo with mass $10^{11}~M_{\odot}$ and a spin parameter of $\sim 0.02$ can form a black hole of $2.3 \times 10^{8}~M_{\odot}$ through GC migration and accretion via tidal disruption events (TDEs). The resulting black hole-to-stellar mass ratio can reach $\sim 0.1$, corresponding to the fraction of GC mass accreted onto the black hole. This mechanism thus provides a plausible explanation for the OMBHs observed by JWST. Furthermore, by combining our model with the halo mass function and the spin-parameter distribution, we construct black hole mass functions that reproduce the number densities of the massive BH candidates UHZ1 and GHZ9 at $z \approx 10$, as well as the abundances of BHs with masses $\gtrsim 10^{8}~\rm{M_\odot}$ at $z \approx 5$ inferred from JWST observations. However, our model overpredicts the abundance of BHs with masses $ < 10^{8}~\rm{M_\odot}$, suggesting that moderately massive, inactive BHs are more frequent.

Figures (7)

• Figure 1: Flow chart of the globular cluster accretion model for the rapid growth of black holes. If galaxies satisfy three conditions, the over-massive black holes form successfully.
• Figure 2: Redshift evolution of the critical spin parameter under which globular clusters can form. Different colors of solid lines represent different halo masses. The black dashed line indicates the peak value of a halo spin parameter distribution Bullock01.
• Figure 3: Migration time of globular clusters into a galactic center as a function of redshift. Different colors of solid lines represent different halo masses with $\lambda=0.03$. The black dotted line shows the cosmic age at specific redshifts.
• Figure 4: Critical star formation efficiencies for the supernova feedback (blue lines), the globular cluster formation (red lines), and migration (green lines) as a function of halo mass. Solid lines show the cases with $\lambda = 0.03$. Dashed and dotted lines represent the cases with $\lambda = 0.02$ and $0.04$, respectively.
• Figure 5: Black hole mass as a function of halo mass. Different line types represent different spin parameters: dot-dashed for $\lambda=0.01$, dashed for $\lambda=0.02$, solid for $\lambda=0.03$, and dotted for $\lambda=0.04$.