UNCOVER Spectroscopy Confirms the Surprising Ubiquity of Active Galactic Nuclei in Red Sources at z > 5

Abstract

The James Webb Space Telescope is revealing a new population of dust-reddened broad-line active galactic nuclei (AGN) at redshifts z ≳ 5. Here we present deep NIRSpec/Prism spectroscopy from the Cycle 1 Treasury program Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) of 15 AGN candidates selected to be compact, with red continua in the rest-frame optical but with blue slopes in the UV. From NIRCam photometry alone, they could have been dominated by dusty star formation or an AGN. Here we show that the majority of the compact red sources in UNCOVER are dust-reddened AGN: 60% show definitive evidence for broad-line Hα with a FWHM > 2000 km s −1, 20% of the current data are inconclusive, and 20% are brown dwarf stars. We propose an updated photometric criterion to select red z > 5 AGN that excludes brown dwarfs and is expected to yield >80% AGN. Remarkably, among all z phot > 5 galaxies with F277W – F444W > 1 in UNCOVER at least 33% are AGN regardless of compactness, climbing to at least 80% AGN for sources with F277W – F444W > 1.6. The confirmed AGN have black hole masses of 107–109 M ⊙. While their UV luminosities (−16 > M UV > −20 AB mag) are low compared to UV-selected AGN at these epochs, consistent with percent-level scattered AGN light or low levels of unobscured star formation, the inferred bolometric luminosities are typical of 107–109 M ⊙ black holes radiating at ∼10%–40% the Eddington limit. The number densities are surprisingly high at ∼10−5 Mpc−3 mag−1, 100 times more common than the faintest UV-selected quasars, while accounting for ∼1% of the UV-selected galaxies. While their UV faintness suggests they may not contribute strongly to reionization, their ubiquity poses challenges to models of black hole growth.

Discovery of Merging Twin Quasars at z = 6.05

“…The spectral properties mentioned above suggest the presence of quasars in both sources. The widths of the Lyα broad components exceed the common threshold of quasar classification, v FWHM = 500-1000 km s −1 (e.g., Schneider et al 2010;Pâris et al 2012), and are similar to values found in faint AGNs revealed by JWST spectroscopy of EoR galaxies (e.g., Greene et al 2024;Harikane et al 2023;Maiolino et al 2023). These values are found at the lower end of the FWHM distribution of low-z Seyfert 1 galaxies in SDSS (e.g., Hao et al 2005) and of high-z low-luminosity quasars found in SHELLQs.…”

“…In the meantime, the James Webb Space Telescope (JWST) has produced groundbreaking results in the past two years. Its extremely high IR sensitivity has revealed a weak broad component in Balmer emission lines of many high-z galaxies, signaling the presence of low-luminosity active galactic nuclei (AGNs) out to z ∼ 11 (e.g., Greene et al 2024;Maiolino et al 2023). The number density of such AGNs exceeds the extrapolation of the classical QLF by several orders of magnitude (e.g., Harikane et al 2023;Matthee et al 2024), changing the paradigm of SMBH activity happening in the EoR.…”

Exploring Low-mass Black Holes through Tidal Disruption Events in the Early Universe: Perspectives in the Era of the JWST, Roman Space Telescope, and LSST Surveys

“…The BH masses for those JWST-detected faint AGNs are M • ∼ 10 6−8 M e , which is 1-2 orders of magnitude lower than those of previously known quasars at z > 4. Intriguingly, some of the sources have extremely red colors in the rest-frame optical bands, while the rest-frame UV color seems fairly consistent with those of known Lyman-break galaxies at similar redshifts (e.g., Harikane et al 2023;Kocevski et al 2023;Labbe et al 2023;Barro et al 2024;Greene et al 2024;Matthee et al 2024). The so-called "little red dot" sources are interpreted to be in a transition stage from a dust-obscured starburst to an unobscured luminous quasar due to the expelling of gas and dust (Fujimoto et al 2022).…”

A Candidate Supermassive Black Hole in a Gravitationally Lensed Galaxy at Z ≈ 10

“…Notably, Ricarte & Natarajan (2018) predict bolometric luminosity functions for accreting SMBHs at a range of redshifts for the light and heavy seed models. In Figure 4, we compare the Ricarte & Natarajan (2018) model predictions of AGN luminosity functions at z ∼ 10 based on UHZ 1 and GHZ 9 and at z = 6-8.5 using data from Greene et al (2024). The observed AGN number density at z = 6-8.5 exceeds the theoretical expectations for the light and heavy seed models.…”