Draft version June 28, 2021 Typeset using LATEX twocolumn style in AASTeX62 Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). XIV. A Candidate Type-II Quasar at z = 6:1292 Masafusa Onoue,1 Yoshiki Matsuoka,2 Nobunari Kashikawa,3, 4 Michael A. Strauss,5 Kazushi Iwasawa,6 Takuma Izumi,7, 8 Tohru Nagao,2 Naoko Asami,9 Seiji Fujimoto,10, 11 Yuichi Harikane,12, 13 Takuya Hashimoto,14 Masatoshi Imanishi,7, 8 Chien-Hsiu Lee,15 Takatoshi, Shibuya,16 and Yoshiki Toba17, 18, 2 1Max-Planck-Institut f¨urAstronomie, K¨onigstuhl17, D-69117 Heidelberg, Germany 2Research Center for Space and Cosmic Evolution, Ehime University, Matsuyama, Ehime 790-8577, Japan 3Department of Astronomy, School of Science, The University of Tokyo, Tokyo 113-0033, Japan 4Research Center for the Early Universe, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 5Princeton University Observatory, Peyton Hall, Princeton, NJ 08544, USA. 6ICREA and Institut de Ci`enciesdel Cosmos, Universitat de Barcelona, IEEC-UB, Mart´ıi Franqu`es,1, 08028 Barcelona, Spain 7National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan 8Department of Astronomical Science, Graduate University for Advanced Studies (SOKENDAI), 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan 9Seisa University, Hakone-machi, Kanagawa, 250-0631, Japan 10Cosmic Dawn Center (DAWN), Jagtvej 128, DK2200 Copenhagen N, Denmark 11Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, DK2100 Copenhagen Ø, Denmark 12Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8582, Japan 13Department of Physics and Astronomy, University Colege London, Gower Street, London WC1E 6BT, UK 14Tomonaga Center for the History of the Universe (TCHoU), Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan 15NSF's National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA. 16Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan 17Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan 18Academia Sinica Institute of Astronomy and Astrophysics, 11F of Astronomy-Mathematics Building, AS/NTU, No.1, Section 4, Roosevelt Road, Taipei 10617, Taiwan. ABSTRACT 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 unresolved (< 230 km s−1) and prominent (> 1044 erg s−1) Lyα emission lines at absolute 1450 A˚ continuum magnitudes of M1450 ∼ −22 mag. Its rest-frame Lyα equivalent width (EW) is 370±30 A.˚ In the 2 hour Keck/MOSFIRE spectrum in Y band, the high-ionization Civ λλ1548,1550 doublet emission +20 −1 +6 ˚ line was clearly detected with FWHM = 120−20 km s and a total rest-frame EW of 37−5 A. We also report the detection of weak continuum emission, and the tentative detection of Oiii]λλ1661; 1666 in the 4 hour J band spectrum. Judging from the UV magnitude, line widths, luminosities, and EWs of Lyα 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 arXiv:2106.13807v1 [astro-ph.GA] 25 Jun 2021 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. Keywords: dark ages, reionization { quasars: general { quasars: individual (HSC J142331.71−001809.1) 1. INTRODUCTION [email protected] The last two decades have yielded remarkable success in the discovery of unobscured quasars in the reioniza- 2 Onoue et al. tion epoch (z > 6). Thanks to the advent of wide-field different viewing angles to the central accretion disks surveys such as the Sloan Digital Sky Survey (e.g., Jiang (e.g., Antonucci 1993; Urry & Padovani 1995). In this et al. 2016) and Pan-Starrs1 (e.g., Ba~nadoset al. 2016), framework, type-II AGNs are observed when the ob- more than 200 z > 5:7 quasars have been reported, servers' lines of sight are obscured by optically thick with redshifts up to z ∼ 7:6 (Ba~nadoset al. 2018; Yang dusty material that blocks the nuclear emission at . 10 et al. 2020; Wang et al. 2021). Most of the luminous pc from the central SMBHs. An alternative model sug- 46 z > 6 quasars (with bolometric luminosity Lbol > 10 gests that the obscured AGNs constitute a transitional erg s−1) are powered by billion-solar-mass supermassive phase during a gas-rich galaxy major merger (e.g., Hop- black holes (SMBHs), which is remarkable given the fact kins et al. 2006); obscured AGNs appear when SMBHs that the universe is less than one billion years old at first ignite, surrounded by dust in the host galaxy gen- z > 6. How such massive SMBHs could form at z > 6 erated by a merger-induced starburst. In these mod- has been the focus of active discussion in the literature els, one predicts a population of dust-reddened broad- (Inayoshi et al. 2020, for a recent review). line quasars before the strong radiation pressure from However, these optical (rest-frame UV) quasar sur- the AGN completely expels the surrounding dust. This veys have not been sensitive to obscured quasars. Only modestly obscured population, so-called red quasars and a handful of candidate type-II active galactic nuclei dust-obscured galaxies, have been observed over a broad (AGNs) are known at z & 6. They have been identi- range of redshift (e.g., Richards et al. 2003; Urrutia et fied as either radio galaxies (Saxena et al. 2018) or com- al. 2008; Glikman et al. 2012; Assef et al. 2013; Ross et panion X-ray sources of luminous unobscured quasars al. 2015; Toba et al. 2018; Kato et al. 2020). Theoretical (Connor et al. 2019; Vito et al. 2019) 1. Luminous type- studies have recently suggested that the initial intense II AGNs whose bolometric luminosities are high enough growth of SMBHs at high redshift is accompanied by to be classified as quasars have been identified at z < 4 high column-density gas and dust in the host galaxies, through observations at various wavelength ranges: op- and such host-scale gas and dust can significantly con- tical (e.g., Zakamska et al. 2003; Alexandroff et al. 2013; tribute to the observed optical depth (Trebitsch et al. Yuan et al. 2016), infrared (e.g., Stern et al. 2012; Lacy 2019; Davies et al. 2019; Ni et al. 2020). Thus there is et al. 2015; Glikman et al. 2018), X-ray (e.g., Stern et al. particular interest in identifying red and obscured AGN 2002; Szokoly et al. 2004), and radio (e.g., van Breugel at high redshift, because those type-II objects may pro- et al. 1999; De Breuck et al. 2000b). The intrinsic lumi- vide clues to the initial growth of SMBHs as well as dust nosities and degree of obscuration depend strongly on and chemical enrichment in the host galaxies. the selection criteria, making it challenging to compare results from different samples. Yet, at z < 0:8 Reyes 1.1. Narrow-Line Population in the z = 6{7 et al.(2008) suggest that type-II quasars are as abun- Low-Luminosity Quasar Sample dant as type-I quasars at fixed [Oiii] λ5007 luminosity A 1000 deg2-class optical survey with the Hyper (L > 108:3L ). Sensitive X-ray observations also [OIII] Suprime-Cam (Miyazaki et al. 2018) mounted on the showed that the obscured (N > 1023 cm−2) fraction H 8.2m Subaru telescope (Aihara et al. 2018) has enabled is > 60% at 3 < z < 6, increasing from lower redshift the deepest investigation to date of the faint end of the (Ueda et al. 2003, 2014; Vito et al. 2018). Those high- quasar luminosity function at high redshift. With this redshift obscured type-II quasars and AGNs are useful survey, the Subaru High-z Exploration Low-Luminosity probes of the metal enrichment of host galaxies, because Quasar (SHELLQs) project has found 93 quasars at their narrow emission lines trace host-scale ionized gas, 5:7 ≤ z ≤ 7:1 in the low-luminosity range (down to unlike broad emission lines from the nuclear region. Ra- bolometric luminosity L ∼ 1045 erg s−1, or 1450 A˚ dio galaxy observations have found no significant red- bol absolute magnitude M ∼ −22) (Matsuoka et al. shift evolution of the narrow-line-region metallicity up 1450 2016, 2018a,b, 2019a,b). Those objects were selected to z ∼ 5 (Nagao et al. 2006; Matsuoka et al. 2009, 2011; with a standard color cut that makes use of red i − z Maiolino & Mannucci 2019). and z −y colors of z ∼ 6 and z ∼ 7 quasars, respectively. Obscuration of AGNs can occur at different scales In the SHELLQs project, quasar candidates are required within the host galaxies. The standard AGN unifica- to be point sources in their HSC images (typical see- tion models explain the two types of AGNs as due to ing of 0:7 arcsecond in the z and y bands). Their UV magnitudes were derived by extrapolating the observed 1 Vito et al.(2021) recently reported no significant X-ray emis- optical continua (rest-frame λrest ≈ 1220{1350A)˚ to sion from Vito et al.(2019)'s z = 6:515 source with deeper follow- rest-frame 1450A,˚ assuming the fiducial power-law slope up observations with Chandra.