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

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.