Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). XIV. A Candidate Type-II Quasar at z=6.1292

TL;DR

This paper reports the discovery and detailed spectroscopic analysis of a candidate type-II quasar at z=6.1292, suggesting it may be an early universe analog of classical AGNs or a new type of AGN/galaxy composite.

Contribution

First detailed spectroscopic characterization of a high-redshift candidate type-II quasar, expanding understanding of AGN properties during reionization.

Findings

Detection of strong Lyman alpha and CIV emission lines.

Evidence suggesting a type-II AGN nature at high redshift.

Comparison with intermediate-redshift type-II AGNs and CIV emitters.

Abstract

We present deep Keck/MOSFIRE near-infrared spectroscopy of a strong Lyman alpha emitting source at z=6.1292, HSC J142331.71-001809.1, which was discovered by the SHELLQS program from imaging data of the Subaru Hyper Suprime-Cam (HSC) survey. This source is one of five objects that show unresolved (<230 km s-1) and prominent (>10^44 erg s-1) Lyman alpha emission lines at absolute 1450 angstrom continuum magnitudes of M1450~-22 mag. Its rest-frame Lyman alpha equivalent width (EW) is 370+/-30 angstrom. In the 2 hour Keck/MOSFIRE spectrum in Y band, the high-ionization CIV 1548,1550 doublet emission line was clearly detected with FWHM =120+/-20 km s-1 and a total rest-frame EW of 37-5+6 angstrom. We also report the detection of weak continuum emission, and the tentative detection of OIII] 1661,1666 in the 4 hour J band spectrum. Judging from the UV magnitude, line widths, luminosities, and EWs of Lyman alpha and CIV, we suggest that this source is a reionization-era analog of classical type-II AGNs, although there is a possibility that it represents a new population of AGN/galaxy composite objects in the early universe. We compare the properties of J1423-0018 to intermediate-redshift type-II AGNs and CIV emitters seen in z=6-7 galaxy samples. Further observations of other metal emission lines in the rest-frame UV or optical, and X-ray follow-up observations of the z=6-7 narrow-line quasars are needed for more robust diagnostics and to determine their nature.