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

TL;DR

Problem: In the Epoch of Reionization, two AGN populations appear distinct—the UV-bright, unobscured quasars and mildly obscured, low-luminosity AGNs—leaving a gap for a bridging population. Approach: JWST/NIRSpec follow-up of 13 rest-frame UV-selected Lyalpha emitters with Lyalpha luminosity above $10^{43}$ erg s$^{-1}$ reveals that 7 of 11 show a broad component in H I Balmer and He I lines but not in [O III], indicating UV-obscured broad-line quasars. Key results: the broad-line objects have intrinsic luminosities consistent with UV-obscured counterparts of luminous EoR quasars, hosting black holes with masses around $10^{7.8-9.1}$ Msun accreting from sub-Eddington to Eddington rates; many sources are unresolved and lie near little red dots with blue rest-UV and red rest-optical colors, and show dust extinction in the range $0 < A_V < 3$. The inferred AGN number density among similarly luminous Lyalpha emitters is greater than a few times $10^{-8}$ Mpc$^{-3}$, comparable to classical quasars. Significance: reveals a substantial, previously overlooked obscured AGN population in the EoR, bridging AGN demographics and informing SMBH growth models; demonstrates JWST's capability to uncover UV-obscured quasars missed by past rest-UV surveys.

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.

Figures (1)

• Figure 1: Redshifts and rest-UV absolute magnitudes of (mostly) luminous quasars in the literature (open squares), SHELLQs low-luminosity quasars (grey dots), SHELLQs galaxies with Ly$\alpha$ luminosity $L_{\rm Ly\alpha} > 10^{43}$ erg s$^{-1}$ (red dots), and JWST BHEs harikane23kokorev23ubler23furtak24greene24juodzbalis24juodzbalis25kocevski24maiolino24matthee24schindler24tripodi24wang24bakins25lin25naidu25_lrdtaylor25 at $z > 5.5$. The 13 JWST targets of this work are marked by the open circles, and those with broad line detections (see text) are marked by the larger open circles. The $M_{\rm UV}$ values are as measured, i.e., not corrected for dust extinction.