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 Ly 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 narrow (FWHM < 500 km s(-1)) and prominent (Ly alpha > 1044 erg s(-1)) Ly alpha emission lines at absolute 1450 angstrom continuum magnitudes of M-1450 similar to-22 mag. Its rest-frame Ly alpha equivalent width (EW) is 370 +/- 30 angstrom. In the 2 hr Keck/MOSFIRE spectrum in Y band, the high-ionization C IV lambda lambda 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) angstrom. We also report the detection of weak continuum emission, and the tentative detection of O III]lambda lambda 1661,1666 in the 4 hr J-band spectrum. Judging from the UV magnitude, line widths, luminosities, and EWs of Ly alpha 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.

Automated summary

In this study, deep Keck/MOSFIRE near-infrared spectroscopy was conducted on a strong Ly alpha emitting source at z = 6.1292, revealing narrow and prominent Ly alpha emission lines as well as other emission lines. The source was compared to quasars and the nature of this source was discussed in detail, suggesting it could represent a new population of objects in the early universe. More observations are needed for further diagnostics and understanding.