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

We present deep Keck/MOSFIRE near-infrared spectroscopy of a strong Lyα 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_Lyα > 10⁴⁴ erg s^-1) Lyα emission lines at absolute 1450 Å continuum magnitudes of M₁₄₅₀ ~ -22 mag. Its rest-frame Lyα equivalent width (EW) is 370 ± 30 Å. In the 2 hr Keck/MOSFIRE spectrum in Y band, the high-ionization C IV λλ1548,1550 doublet emission line was clearly detected with $\mathrm{FWHM}={120}_{-20}^{+20}$ 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]λλ1661,1666 in the 4 hr J-band spectrum. Judging from the UV magnitude, line widths, luminosities, and EWs of Lyα 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.