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- Title
Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). III. Star formation properties of the host galaxies at z ≳ 6 studied with ALMA.
- Authors
Takuma IZUMI; Masafusa ONOUE; Hikari SHIRAKATA; Tohru NAGAO; Kotaro KOHNO; Yoshiki MATSUOKA; Masatoshi IMANISHI; STRAUSS, Michael A.; KASHIKAWA, Nobunari; SCHULZE, Andreas; SILVERMAN, John D.; Seiji FUJIMOTO; Yuichi HARIKANE; Yoshiki TOBA; Hideki UMEHATA; Kouichiro NAKANISHI; GREENE, Jenny E.; Yoichi TAMURA; Akio TANIGUCHI; Yuki YAMAGUCHI
- 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) x 108 L⊙ and LFIR = (1.2-2.0) x 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) x 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) x 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.
- Subjects
LUMINOSITY; STAR formation; QUASARS; GALAXIES; STELLAR atmospheres
- Publication
Publications of the Astronomical Society of Japan, 2018, Vol 70, Issue 3, p36-1
- ISSN
0004-6264
- Publication type
Article
- DOI
10.1093/pasj/psy026