SHELLQs. Bridging the Gap: JWST Unveils Obscured Quasars in the Most Luminous Galaxies at z > 6

Yoshiki Matsuoka, Masafusa Onoue, Kazushi Iwasawa, Kentaro Aoki, Michael A. Strauss, John D. Silverman, Xuheng Ding, Camryn L. Phillips, Masayuki Akiyama, Junya Arita, Masatoshi Imanishi, Takuma Izumi, Nobunari Kashikawa, Toshihiro Kawaguchi, Satoshi Kikuta, Kotaro Kohno, Chien-Hsiu Lee, Tohru Nagao, Ayumi Takahashi, Yoshiki Toba

First listed 2025-05-07 | Last updated 2025-12-09

Abstract

The unprecedented sensitivity of the James Webb Space Telescope (JWST) has uncovered a surprisingly abundant population of mildly obscured, low-luminosity active galactic nuclei (AGNs) in the epoch of reionization (EoR). However, the link between these objects and classical unobscured quasars remains a mystery. Here we report the discovery of obscured quasars hosted by the most luminous galaxies at z > 6, possibly bridging the gap between the two AGN populations. The 13 objects presented here were originally selected from a rest-frame ultraviolet (UV) imaging survey over >1000 deg2, and were known to have luminous (>10^{43} erg/s) narrow Ly-alpha emission. With JWST/NIRSpec follow-up observations, we found that 7 out of 11 objects with narrow Ly-alpha exhibit a broad component in H I Balmer lines and He I lines, but not in [O III] and other forbidden lines. Mild dust obscuration (0 < Av < 3) is inferred from the Balmer decrements. The estimated intrinsic luminosities suggest that our broad line (BL) objects are the long-sought UV-obscured counterparts of luminous quasars in the EoR. They host supermassive black holes (SMBHs) with masses 10^(7.8-9.1) Msun, undergoing sub-Eddington to Eddington accretion. Most of the BL objects are spatially unresolved, and are close to "little red dots" with their blue rest-UV and red rest-optical colors. We estimate the AGN number density among similarly luminous Ly-alpha emitters to be larger than 2 x 10^(-8) /Mpc3. This density is comparable to that of classical quasars with similar continuum luminosities, suggesting that a substantial fraction of active SMBHs are obscured in the EoR and have been overlooked in past rest-UV surveys.

Short digest

From the SHELLQs wide-field UV survey, the team followed 13 z>6, Lyα-luminous galaxies with JWST/NIRSpec and uncovered obscured quasars. Seven of eleven narrow‑Lyα systems show broad H I Balmer and He I components—but not in [O III]—with Balmer decrements indicating mild obscuration (0<Av<3), identifying them as UV‑obscured counterparts of luminous EoR quasars. Virial lines imply MBH≈10^(7.8–9.1) Msun with sub‑Eddington to Eddington accretion; most BL sources are unresolved and display LRD‑like blue‑UV/red‑optical colors. The AGN incidence among similarly bright Lyα emitters (≥2×10^−8 Mpc^−3) is comparable to classical quasar densities, implying a substantial obscured SMBH population previously missed by UV‑bright surveys.

Key figures to inspect

• Figure 1: Locate the 13 targets relative to classical quasars and JWST BHEs; note which open circles (our sample) are flagged as BL detections to see the luminosity/redshift regime that bridges LRD/BHEs and UV‑bright QSOs.
• Figure 2: Inspect G01–G05 NIRSpec spectra for broad Hβ/Hγ and He I atop narrow lines; use the insets to gauge Balmer decrements that yield Av≈0–3 and confirm the lack of broad [O III].
• Figure 3: For G06–G10, verify BL profiles and the absence of broad forbidden lines; in G07, check the weak 4924 Å and 5756 Å features (possible Fe II/iron lines) as flagged by arrows.
• Figure 4: For G11, Q01, Q02, compare BL signatures and search for the unidentified 5161 Å (and 5270 Å) lines in Q01/Q02 (candidate [Fe VII]); cross‑check that forbidden lines remain narrow while Balmer/He I are broad.
• Line-profile panels (Figs. 2–4): Measure FWHM of Balmer/He I vs [O III] to see the BLR–NLR dichotomy that underpins the SMBH mass and obscuration inferences.