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

TL;DR

JWST observations reveal a significant population of obscured quasars at z > 6, bridging the gap between obscured and unobscured AGNs during the epoch of reionization, and indicating many SMBHs are hidden from previous surveys.

Contribution

This study uncovers the existence of obscured quasars in luminous high-redshift galaxies, providing new insights into AGN populations during the epoch of reionization.

Findings

Obscured quasars host SMBHs with masses 10^{7.8-9.1} Msun.

The density of obscured AGNs is comparable to that of classical quasars.

Many active SMBHs at z > 6 are obscured and previously overlooked.

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.