Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). III. Star formation properties of the host galaxies at z ∼ 6 studied with ALMA

Takuma Izumi, Masafusa Onoue, Hikari Shirakata, Tohru Nagao, Kotaro Kohno, Yoshiki Matsuoka, Masatoshi Imanishi, Michael A. Strauss, Nobunari Kashikawa, Andreas Schulze, John D. Silverman, Seiji Fujimoto, Yuichi Harikane, Yoshiki Toba, Hideki Umehata, Kouichiro Nakanishi, Jenny E. Greene, Yoichi Tamura, Akio Taniguchi, Yuki YamaguchiTomotsugu Goto, Yasuhiro Hashimoto, Soh Ikarashi, Daisuke Iono, Kazushi Iwasawa, Chien Hsiu Lee, Ryu Makiya, Takeo Minezaki, Ji Jia Tang

    Research output: Contribution to journalArticlepeer-review

    24 Citations (Scopus)

    Abstract

    We present our ALMA Cycle 4 measurements of the [C II] emission line and the underlying far-infrared (FIR) continuum emission from four optically low-luminosity (M1450 > -25) quasars at z ≳ 6 discovered by the Subaru Hyper Suprime Cam (HSC) survey. The [C II] line and FIR continuum luminosities lie in the ranges L[C II] = (3.8-10.2) × 108 L· and LFIR = (1.2-2.0) × 1011 L·, which are at least one order of magnitude smaller than those of optically-luminous quasars at z ≳ 6. We estimate the star formation rates (SFRs) of our targets as ≲ 23-40 M· yr-1. Their line and continuum-emitting regions are marginally resolved, and found to be comparable in size to those of optically-luminous quasars, indicating that their SFR or likely gas mass surface densities (key controlling parameter of mass accretion) are accordingly different. The L[C II]/LFIR ratios of the hosts, ≲ (2.2- 8.7) × 10-3, are fully consistent with local star-forming galaxies. Using the [C II] dynamics, we derived their dynamical masses within a radius of 1.5-2.5 kpc as ≲ (1.4-8.2) × 1010 M·. By interpreting these masses as stellar ones, we suggest that these faint quasar hosts are on or even below the star-forming main sequence at z ∼ 6, i.e., they appear to be transforming into quiescent galaxies. This is in contrast to the optically-luminous quasars at those redshifts, which show starburst-like properties. Finally, we find that the ratios of black hole mass to host galaxy dynamical mass of most of the low-luminosity quasars, including the HSC ones, are consistent with the local value. The mass ratios of the HSC quasars can be reproduced by a semi-analytical model that assumes merger-induced black hole host galaxy evolution.

    Original languageEnglish
    Article number36
    JournalPublications of the Astronomical Society of Japan
    Volume70
    Issue number3
    DOIs
    Publication statusPublished - 2018 Jun 1

    Keywords

    • galaxies: ISM
    • galaxies: high-redshift
    • galaxies: quasars
    • galaxies: starburst
    • quasars: general

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science

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