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

TL;DR

This work uses JWST-detected dust-reddened AGNs (LRDs) to probe black hole growth at $z\gtrsim 4-8$, revealing an abundant obscured AGN population that drives a high BH accretion rate density. By applying the Soltan-Paczy\'nski framework to LRD bolometric luminosities, the study finds that a canonical radiative efficiency of $\epsilon_{\rm rad}=0.1$ cannot reconcile the observed BH mass density with the accreted mass, instead requiring $\epsilon_{\rm rad}\gtrsim 0.2$–0.3 and implying rapid spins with $a_\bullet \approx 0.96$–$0.996$ under thin-disk accretion. The analysis also places stringent upper bounds on host stellar masses, yielding $\mathscr{F}<0.3\,f_\star$ at $4.5<z<6.5$ and $\mathscr{F}<0.04\,f_\star$ at $6.5<z<8.5$, and showing $M_\bullet$–$M_\star$ relations in LRDs are elevated relative to local trends. Together, these results support a scenario where dusty, dense environments enable prolonged, coherent accretion and rapid BH spins during reionization, with potential jet and multi-messenger imprints detectable by future observatories.

Abstract

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 $z\sim 6$. 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 $z\simeq 4-5$, 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 $>3σ$ 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.

Figures (6)

• Figure 1: Bolometric AGN luminosity functions at $4.5<z<6$ (left) and $6.5<z<8.5$ (right). The luminosity function data obtained from different surveys are shown: the rest-UV-selected quasars Niida_2020Matsuoka_2023, the X-ray selected AGNs Ueda_2014, and dust-reddened AGNs reported as "little red dots (LRDs)" identified with JWST photometry and slitless spectroscopy Matthee_2024Kokorev_2024Greene_2024Akins_2024. The bright end slope of the LRD luminosity function is consistent with $\Phi \propto L_{\rm bol}^{-1}$ at both redshifts.
• Figure 2: The cosmic BH accretion rate density (BHAD) as a function of redshift. Each data point and curve represent BHADs estimated under the assumption of a 10% radiative efficiency ($\epsilon_{\rm rad}=0.1$) for the three different populations, including LRDs Matthee_2024Kokorev_2024Greene_2024Akins_2024, X-ray selected AGNs including Compton-thick populations Aird_2015Ananna_2019Pouliasis_2024, and mid-infrared selected AGNs Delvecchio_2014. For comparison, the cosmic SFRD scaled by a factor of 3,000 is overlaid Harikane_2022a. The BHAD attributed to LRDs remains significantly dominant at $z>6$.
• Figure 3: Left: Cosmic evolution of the BH mass density in a comoving volume. At $z\sim 5$, the BH mass density is derived from the integration of the BHMF for LRDs (red symbols). From this point, the mass density grows toward lower redshifts following the BHAD deduced from known AGN populations with a 10% radiative efficiency (solid curve; Ueda_2014) and reaches the density of relic BHs in the nearby universe Shankar_2009. At $z>5$, the cumulative mass accreted to BHs during the LRD phase, $\Delta \rho_\bullet \equiv {\rm BHAD}\times \Delta t$ inferred from their bolometric luminosity function over a time span $\Delta t$ for given redshift range based on the COSMOS-Web (magenta) and the other surveys (blue), assuming a 10% radiative efficiency, substantially exceed the observed mass density at $z\simeq 5$ as well as the predictions from a BH growth model calibrated with UV and X-ray selected AGN luminosity function Li_2023b. Data for LRDs are derived from luminosity functions and BH mass estimates provided in the literature (open symbols, Matthee_2024Greene_2024Kokorev_2024Akins_2024) and the mean values for each group (filled symbols). Right: Summary of the BH mass density and the cumulative mass density during the LRD phase assuming a 10% radiative efficiency. Shaded areas indicate the BH mass density $\rho_\bullet$ at $z\simeq 5$ (red) and the cumulative mass density accrued during the LRD stage calculated from the COSMOS-Web (magenta) and the other surveys (blue). The total sum of $\Delta \rho_\bullet$ over the entire redshift range in each data group is shown with a star symbol.
• Figure 4: Stellar mass density in galaxies hosting LRDs at various redshifts, calculated using Eq. (\ref{['eq:Mstar']}) and assuming $\mathscr{F}(\equiv f_{\rm IMF}f_L)=1.0$ (open symbols) at two redshift ranges of $4.5<z<6.5$ and $6.5<z<8.5$. For comparison, the stellar mass function derived from the DM halo mass function at $5\leq z \leq 8$ is shown with a 100% star formation efficiency. An upper bound of the stellar mass constrains $\mathscr{F}<0.3 f_\star$ for $4.5<z<6.5$ and $\mathscr{F}<0.04 f_\star$ for $6.5<z<8.5$ (filled symbols).
• Figure 5: $M_\bullet - M_\star$ distribution for high-redshift AGNs, including LRDs, JWST-detected unobscured ANGs at $z=4-8$ (purple, Maiolino_2023_JADES; green, Harikane_2023_agn; cyan, Stone_2024; and blue, Ding_2023), and quasars identified in ground-based surveys Izumi_2021. For the LRDs at $4.5<z<6.5$ (red) and $6.5<z<8.5$ (orange), we derive the upper bound of the stellar mass based on the dust-corrected continuum flux measured by Greene_2024 using Eqs. (\ref{['eq:Mstar']}) and (\ref{['eq:FF']}). Additionally, a $z=8.5$ LRD with broad H$\beta$ emission, for which the stellar mass is constrained by ALMA non-detections, is overlaid Kokorev_2023. Two different mass correlations are overlaid: the local relationship Kormendy_Ho_2013 and the JWST-detected AGNs Pacucci_2023.