Birth of Rapidly Spinning, Overmassive Black Holes in the Early Universe

Kavli Affiliate: Kohei Inayoshi

| First 5 Authors: Kohei Inayoshi, Kohei Ichikawa, , ,

| Summary:

The James Webb Space Telescope (JWST) has unveiled numerous massive black
holes (BHs) in faint, broad-line active galactic nuclei (AGNs). The discovery
highlights the presence of dust-reddened AGN populations, referred to as little
red dots (LRDs), more abundant than X-ray selected AGNs, which are less
influenced by obscuration. This finding indicates that the cosmic growth rate
of BHs within this population does not decrease but rather increases at higher
redshifts beyond $zsim 6$. The BH accretion rate density deduced from their
luminosity function is remarkably higher than that from other AGN surveys in
X-ray and infrared bands. To align the cumulative mass density accreted to BHs
with the observed BH mass density at $zsimeq 4-5$, as derived from the
integration of the BH mass function, the radiative efficiency must be doubled
from the canonical 10% value, achieving significance beyond the $>2sigma$
confidence level. This suggests the presence of rapid spins with 96% of the
maximum limit among these BHs, maintained by prolonged mass accretion instead
of chaotic accretion with randomly oriented inflows. Moreover, we derive an
upper bound for the stellar mass of galaxies hosting these LRDs, ensuring
consistency with galaxy formation in the standard cosmological model, where the
host stellar mass is limited by the available baryonic reservoir. Our analysis
gives a lower bound for the BH-to-galaxy mass ratio that exceeds the typical
value known in the nearby universe and aligns with that for JWST-detected
unobscured AGNs. Accordingly, we propose a hypothesis that the dense, dust-rich
environments within LRDs facilitate the emergence of rapidly spinning and
overmassive BH populations during the epoch of reionization. This scenario
predicts a potential association between relativistic jets and other
high-energy phenomena with overmassive BHs in the early universe.

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