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

**Authors:** Kohei Inayoshi, Kohei Ichikawa  
**arXiv:** [2402.14706](https://arxiv.org/abs/2402.14706)  
**Date:** 2024-02-22  
**Summary page:** https://www.summarizepaper.com/en/paper/2402.14706v1/

## Key points

- The James Webb Space Telescope (JWST) discovered numerous massive black holes in faint, broad-line active galactic nuclei (AGNs).
- Little red dots (LRDs) are more abundant than X-ray selected AGNs and challenge previous assumptions about the cosmic growth rate of black holes.
- The luminosity function of LRDs suggests a significantly higher black hole accretion rate density compared to other AGN surveys in X-ray and infrared bands.
- Doubling the radiative efficiency from the canonical 10% value is proposed to reconcile cumulative mass density accreted to black holes with observed BH mass density at $z\simeq 4-5$.
- Rapidly spinning black holes with spins reaching 96% of the maximum limit are maintained by prolonged mass accretion rather than chaotic accretion with randomly oriented inflows.

## Summary

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The James Webb Space Telescope (JWST) has made a groundbreaking discovery by unveiling numerous massive black holes (BHs) in faint, broad-line active galactic nuclei (AGNs). These AGNs are characterized by dust-reddened populations known as little red dots (LRDs), which are found to be more abundant than X-ray selected AGNs. This finding challenges previous assumptions about the cosmic growth rate of BHs, revealing that it actually increases at higher redshifts beyond $z\sim 6$. The luminosity function of these LRDs suggests a significantly higher BH accretion rate density compared to other AGN surveys in X-ray and infrared bands. To reconcile the cumulative mass density accreted to BHs with the observed BH mass density at $z\simeq 4-5$, it is proposed that the radiative efficiency must be doubled from the canonical 10% value. This adjustment reaches significance beyond the $>2\sigma$ confidence level and implies the presence of rapidly spinning BHs with spins reaching 96% of the maximum limit. These high spin rates are maintained by prolonged mass accretion rather than chaotic accretion with randomly oriented inflows. Additionally, an upper bound for the stellar mass of galaxies hosting LRDs has been derived to ensure consistency with galaxy formation models based on available baryonic reservoirs. The analysis also reveals a lower bound for the BH-to-galaxy mass ratio, exceeding typical values known in the nearby universe and aligning with those for JWST-detected unobscured AGNs. Based on these findings, a hypothesis is proposed suggesting that dense, dust-rich environments within LRDs facilitate the emergence of rapidly spinning and overmassive BH populations during the epoch of reionization. This scenario predicts potential associations between relativistic jets and other high-energy phenomena with overmassive BHs in the early universe. Despite some complexities in current analyses, including considerations such as BH mergers and mass estimates, the central conclusion remains valid on a qualitative level. Further observational explorations are needed to refine our understanding of LRD characteristics and provide more precise estimates of their properties.