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We present our ALMA Cycle 4 measurements of the [CII] emission line and theunderlying far-infrared (FIR) continuum emission from four opticallylow-luminosity ($M_{\rm 1450} > -25$) quasars at $z \gtrsim 6$ discovered bythe Subaru Hyper Suprime Cam (HSC) survey. The [CII] line and FIR continuumluminosities lie in the ranges $L_{\rm [CII]} = (3.8-10.2) \times 10^8~L_\odot$and $L_{\rm FIR} = (1.2-2.0) \times 10^{11}~L_\odot$, which are at least oneorder of magnitude smaller than those of optically-luminous quasars at $z\gtrsim 6$. We estimate the star formation rates (SFR) of our targets as$\simeq 23-40~M_\odot ~{\rm yr}^{-1}$. Their line and continuum-emittingregions are marginally resolved, and found to be comparable in size to those ofoptically luminous quasars, indicating that their SFR or likely gas masssurface densities (key controlling parameter of mass accretion) are accordinglydifferent. The $L_{\rm [CII]}/L_{\rm FIR}$ ratios of the hosts, $\simeq(2.2-8.7) \times 10^{-3}$, are fully consistent with local star-forminggalaxies. Using the [CII] dynamics, we derived their dynamical masses within aradius of 1.5-2.5 kpc as $\simeq (1.4-8.2) \times 10^{10}~M_\odot$. Byinterpreting these masses as stellar ones, we suggest that these faint quasarhosts are on or even below the star-forming main sequence at $z \sim 6$, i.e.,they appear to be transforming into quiescent galaxies. This is in contrast tothe optically luminous quasars at those redshifts, which show starburst-likeproperties. Finally, we find that the ratios of black hole mass to host galaxydynamical mass of the most of low-luminosity quasars including the HSC ones areconsistent with the local value. The mass ratios of the HSC quasars can bereproduced by a semi-analytical model that assumes merger-induced blackhole-host galaxy evolution.

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