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

Abstract

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.

NGDEEP Epoch 1: The Faint End of the Luminosity Function at z ∼ 9–12 from Ultradeep JWST Imaging

“…A class of sources with similar properties has been discovered by a number of studies at z = 5-8 using JWST imaging (Furtak et al 2022;Akins et al 2023;Barro et al 2023;Kocevski et al 2023;Labbé et al 2023). The origin of these sources has been attributed to dust-obscured galaxies (Akins et al 2023), SMBH accretion (Furtak et al 2022;Kocevski et al 2023), or evolved stellar populations (Labbé et al 2023). These objects, however, are the first to be observed at z ∼ 9 and m F444W ∼ 28.…”

“…We note that the morphology of NGDEEP 20351 in the F444W filter appears to resemble the diffraction spikes produced by point sources. A class of sources with similar properties has been discovered by a number of studies at z = 5-8 using JWST imaging (Furtak et al 2022;Akins et al 2023;Barro et al 2023;Kocevski et al 2023;Labbé et al 2023). The origin of these sources has been attributed to dust-obscured galaxies (Akins et al 2023), SMBH accretion (Furtak et al 2022;Kocevski et al 2023), or evolved stellar populations (Labbé et al 2023).…”

UNCOVER: The Growth of the First Massive Black Holes from JWST/NIRSpec—Spectroscopic Redshift Confirmation of an X-Ray Luminous AGN at z = 10.1

“…On the other hand, the lack of UV AGN signatures is not unusual. Even in other bona fide broad-line AGN detected by JWST, which interestingly lack X-ray emission despite their Type-1 AGN nature (e.g., Furtak et al 2023c), these systems are also bereft of the typical broad and narrow AGN emission lines blueward of Hβ (e.g., Harikane et al 2023;Kocevski et al 2023;Matthee et al 2023;Labbe et al 2023). Moreover, such objects are not entirely uncommon in the nearby Universe (e.g., NGC 1448; NGC 4945; see Alexander & Hickox 2012;Annuar et al 2017) as even luminous AGN signatures can be swamped by strong star formation and/or extreme obscuration.…”

“…The situation has gotten decidedly more interesting with the launch of JWST. A number of intriguing active galactic nuclei (AGN) candidates have been spectroscopically confirmed at more moderate luminosities (Kocevski et al 2023;Matthee et al 2023), with some discovered at z > 7 (Harikane et al 2022;Furtak et al 2023a;Larson et al 2023;Maiolino et al 2023aMaiolino et al , 2023b. In the absence of other direct indicators of AGN activity (e.g., broadened Balmer emission lines), the most unambiguous identification of AGN activity is through the detection of high-energy X-ray emission.…”

Modeling the JWST High-redshift Galaxies with a General Formation Scenario and the Consistency with the ΛCDM Model

“…The former is characterized by a high stellar mass (  M M log 10) and significant dust obscuration (A v 3 mag), while the latter exhibits lower stellar mass (  M M log 7.5) and minimal obscuration (A v ∼ 0 mag). This perspective has been supported by the confusion between a dusty AGN (z ∼ 5.62) and high-redshift galaxy CEERS 3210 (z ∼ 8.62) (Kocevski et al 2023). To confirm such overdensity, future observations should expand the search field to reduce the effects of CV.…”