Hidden Little Monsters: Spectroscopic Identification of Low-mass, Broad-line AGNs at z > 5 with CEERS

By D. Kocevski, M. Onoue, K. Inayoshi, J. Trump, P. Haro, A. Grazian, Mark Dickinson, S. Finkelstein, J. Kartaltepe, M. Hirschmann and 31 more. Astrophysical Journal Letters, 2023

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We report on the discovery of two low-luminosity, broad-line active galactic nuclei (AGNs) at z > 5 identified using JWST NIRSpec spectroscopy from the Cosmic Evolution Early Release Science (CEERS) survey. We detect broad Hα emission in the spectra of both sources, with FWHM of 2060 ± 290 km s−1 and 1800 ± 200 km s−1, resulting in virial black hole (BH) masses that are 1–2 dex below those of existing samples of luminous quasars at z > 5. The first source, CEERS 2782 at z = 5.242, is 2–3 dex fainter than known quasars at similar redshifts and was previously identified as a candidate low-luminosity AGN based on its morphology and rest-frame optical spectral energy distribution (SED). We measure a BH mass of M BH = (1.3 ± 0.4) × 107 M ⊙, confirming that this AGN is powered by the least massive BH known in the Universe at the end of cosmic reionization. The second source, CEERS 746 at z = 5.624, is inferred to be a heavily obscured, broad-line AGN caught in a transition phase between a dust-obscured starburst and an unobscured quasar. We estimate its BH mass to be in the range of M BH ≃ (0.9–4.7) × 107 M ⊙, depending on the level of dust obscuration assumed. We perform SED fitting to derive host stellar masses, M ⋆, allowing us to place constraints on the BH–galaxy mass relationship in the lowest mass range yet probed in the early Universe. The M BH/M ⋆ ratio for CEERS 2782, in particular, is consistent with or higher than the empirical relationship seen in massive galaxies at z = 0. We examine the narrow emission line ratios of both sources and find that their location on the BPT and OHNO diagrams is consistent with model predictions for moderately low metallicity AGNs with Z/Z ⊙ ≃ 0.2–0.4. The spectroscopic identification of low-luminosity, broad-line AGNs at z > 5 with M BH ≃ 107 M ⊙ demonstrates the capability of JWST to push BH masses closer to the range predicted for the BH seed population and provides a unique opportunity to study the early stages of BH–galaxy assembly.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/e4790114df48e2b248086c745332c64cb6de9a14

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Citations in the corpus

The Complete CEERS Early Universe Galaxy Sample: A Surprisingly Slow Evolution of the Space Density of Bright Galaxies at z ∼ 8.5–14.5 (2023, background): “Such a contribution remains possible, as scattered UV light from a partially obscured AGN could in theory contribute to the observed UV emission ( e.g., Barro et al. 2024; Greene et al. 2024; Kocevski et al. 2023; Labbe et al. 2023 ) , though the resolved nature of the rest-UV emission in these gala”
A Census of Photometrically Selected Little Red Dots at 4 < z < 9 in JWST Blank Fields (2024, methodology): “…spectra revealed clear evidence for broad H α and/or H β emission indicative of actively accreting supermassive black holes (SMBH; Furtak et al. 2023b; Fujimoto et al. 2023a; Greene et al. 2023; Killi et al. 2023; Kocevski et al. 2023; Kokorev et al. 2023a; Matthee et al. 2023; ¨Ubler et al. 2023).”
JWST CEERS and JADES Active Galaxies at z = 4–7 Violate the Local M •–M ⋆ Relation at >3σ: Implications for Low-mass Black Holes and Seeding Models (2023, background): “Note that this set already includes the two sources reported by Kocevski et al. (2023), with CEERS identification numbers 2782 and 746.”
The Rise of Faint, Red Active Galactic Nuclei at z > 4: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields (2025, unknown): “…offset agrees with several previous studies that report similarly overmassive BHs in faint, broad-line AGN identi ﬁ ed by JWST at z > 4 ( e.g., Y. Harikane et al. 2023; D. D. Kocevski et al. 2023; F. Pacucci et al. 2023; R. Maiolino et al. 2024; A. J. Taylor et al. 2024; E. Durodola et al. 2025 ) .”
A JWST/NIRSpec First Census of Broad-line AGNs at z = 4–7: Detection of 10 Faint AGNs with M BH ∼ 106–108 M ⊙ and Their Host Galaxy Properties (2023, methodology): “Among these three sources, CEERS 00746 (CEERS 3210 in Kocevski et al. 2023) and CEERS 00672 show red spectral-energy distributions (SEDs) and large dust attenuation estimated from the Balmer decrement as presented in Section 4.2, indicating that these two sources are dusty red AGNs.”