[Paper Review] Hidden Little Monsters: Spectroscopic Identification of Low-Mass, Broad-Line AGN at $z>5$ with CEERS

D. Kocevski, Masafusa Onoue+35 more|arXiv (Cornell University)|Jan 31, 2023

Galaxies: Formation, Evolution, Phenomena23 citations

TL;DR

The authors report the discovery of two low-luminosity, broad-line AGN at z>5 identified with JWST/NIRSpec in CEERS, measuring black hole masses around a few 10^7 solar masses and placing constraints on BH-galaxy scaling at the lowest masses probed to date.

ABSTRACT

We report on the discovery of two low-luminosity, broad-line AGN at $z>5$ identified using JWST NIRSpec spectroscopy from the CEERS Survey. We detect broad H$α$ emission from both sources, with FWHM of $2038\pm286$ and $1807\pm207$ km s$^{-1}$, resulting in black hole (BH) masses that are 1-2 dex below that of existing samples of luminous quasars at $z>5$. The first source, CEERS 1670 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 rest-frame optical SED. We measure a BH mass of $M_{ m BH}=1.3\pm0.4 imes 10^{7}~M_{\odot}$, 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 3210 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 $M_{ m BH}\simeq 0.9-4.7 imes 10^{7}~M_{\odot}$, depending on the level of dust obscuration assumed. We derive host stellar masses, $M_\star$, allowing us to place constraints on the BH-galaxy mass relationship in the lowest mass range yet probed in the early universe. The $M_{ m BH}/M_\star$ ratio for CEERS 1670, in particular, is consistent with or higher than the empirical relationship seen in massive galaxies at $z=0$. We examine the emission-line ratios of both sources and find that their location on the BPT and OHNO diagrams is consistent with model predictions for low-metallicity AGN with $Z/Z_\odot \simeq 0.2-0.4$. The spectroscopic identification of low-luminosity, broad-line AGN at $z>5$ with $M_{ m BH}\simeq 10^{7}~M_{\odot}$ 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.

Motivation & Objective

• Demonstrate the spectroscopic identification of low-luminosity, broad-line AGN at z>5 using JWST CEERS data.
• Measure broad H-alpha emission and derive virial black hole masses.
• Constrain host galaxy properties and BH–host mass relations at the low-mass end in the early universe.
• Assess emission-line diagnostics (BPT/OHNO) to understand ISM conditions in these AGN.
• Discuss implications for the detectable AGN population and black hole seeding in the early cosmos.

Proposed method

• Obtain JWST CEERS NIRSpec G395M (R~1000) spectra and NIRCam imaging.
• Identify broad H-alpha emission and decompose with narrow+broad Gaussian components.
• Estimate black hole masses using the Greene & Ho (2005) broad H-alpha-based virial relation.
• Fit host galaxy SEDs with photometry to estimate stellar masses M_star.
• Place emission-line ratios on BPT and OHNO diagrams and compare with MAPPINGS V photoionization models.
• Assess potential dust obscuration and extinction impacts on derived quantities.

Experimental results

Research questions

• RQ1 Can we identify low-luminosity, broad-line AGN at z>5 with JWST CEERS spectroscopy?
• RQ2 What are the black hole masses and Eddington ratios of these z>5, low-luminosity AGN?
• RQ3 How do the host galaxy properties relate to black hole masses at the low-mass end in the early universe?
• RQ4 What do the narrow-line ratios imply about metallicity and ISM conditions of high-z AGN?

Key findings

• Two broad-line AGN at z>5 (CEERS 1670 at z=5.242 and CEERS 3210 at z=5.624) show broad H-alpha with FWHM ~ 2060 and 1800 km/s respectively.
• Virial BH masses are M_BH ≈ 1.3×10^7 M⊙ for CEERS 1670 and M_BH ≈ 0.9–4.7×10^7 M⊙ for CEERS 3210.
• CEERS 1670 is 2–3 dex fainter than known quasars at similar redshifts, highlighting a population of low-luminosity, low-mass BHs in the early universe.
• Host stellar masses are estimated (M_star ≈ 6×10^9 M⊙ for CEERS 1670; higher for CEERS 3210 depending on modeling), enabling BH–host mass constraints at the low-mass end.
• Emission-line diagnostics (BPT and OHNO) place both sources in high-ionization, moderately low-metallicity regimes (Z/Z⊙ ≈ 0.2–0.4), consistent with low-metallicity AGN at high z.
• Neither source is detected in deep X-rays; their broad-line signatures provide a path to identify such AGN when X-ray selection is ineffective.