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

Abstract

We present deep Keck/MOSFIRE near-infrared spectroscopy of a strong Ly α 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 narrow (FWHM < 500 km s −1 ) and prominent ( L Ly α > 10 44 erg s −1 ) Ly α emission lines at absolute 1450 Å continuum magnitudes of M 1450 ∼ −22 mag. Its rest-frame Ly α equivalent width (EW) is 370 ± 30 Å. In the 2 hr Keck/MOSFIRE spectrum in Y band, the high-ionization C iv λλ 1548,1550 doublet emission line was clearly detected with FWHM = 120 − 20 + 20 km s −1 and a total rest-frame EW of 37 − 5 + 6 Å. We also report the detection of weak continuum emission, and the tentative detection of O iii] λλ 1661,1666 in the 4 hr J -band spectrum. Judging from the UV magnitude, line widths, luminosities, and EWs of Ly α and C iv , 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 C iv emitters seen in z = 6–7 galaxy samples. Further observations of other metal emission lines in the rest-frame UV or optical, along with X-ray follow-up observations of the z = 6–7 narrow-line quasars, are needed for more robust diagnostics and to determine their nature.