The outstanding mass growth of supermassive black holes (SMBHs) at the epoch of reionisation and its relation to the concurrent growth of their host galaxies poses challenges to theoretical models aimed at explaining how these systems formed on short timescales (<1 Gyr). To trace the average evolutionary paths of quasi-stellar objects (QSOs) and their host galaxies in the plane of BH mass to host mass (M dyn ), we compare the star formation rate (SFR), derived from the accurate estimate of the dust temperature and the dust mass (T dust , M dust ) based on infrared and sub-millimeter (sub-mm) spectral energy distribution (SED), with the BH accretion rate, derived from L bol based on X-ray and optical and ultraviolet SED. To this aim, we analysed a deep ALMA observation of the submm continuum, [CII], and H 2 O of the z ∼ 6 QSO J2310+1855 with a resolution of 900 pc, which enabled a detailed study of dust properties and cold gas kinematics. We performed an accurate SED analysis obtaining a dust temperature of T dust = 71 ± 4 K, dust mass M dust = (4.4 ± 0.7) × 10 8 M , and total far-infrared luminosity of L TIR = 2.5 +0.6 −0.5 × 10 13 L . The implied active galactic nuclei (AGN) -corrected SFR = 1240 +310 −260 M yr −1 is a factor of 2 lower than previously reported for this QSO. We measured a gas-to-dust ratio of GDR= 101 ± 20. The dust continuum and [CII] surface brightness profiles are spatially extended out to r ∼ 6.7 kpc and r ∼ 5 kpc, respectively, with half-light radii of 0.9 and 1.1 kpc for the dust and gas, respectively. The derived gas surface density, Σ gas , and star formation rate density, Σ SFR , place the J2310+1855 host galaxy above the Kennicutt-Schmidt relation. We derived a best estimate of the dynamical mass M dyn = 5.2 × 10 10 M within r = 1.7 kpc based on a dynamical model of the system with a rotating disk inclined at i = 25 deg. The Toomre parameter profile across the disk is Q gas ∼ 3 and implies that the disk is unstable. We found that SFR/M dyn > ṀBH /M BH , suggesting that AGN feedback might be efficiently acting to slow down the SMBH accretion, while stellar mass assembly is still vigorously taking place in the host galaxy. In addition, we were also able to detect high-velocity emission on the red and blue sides of the [CII] emission line that is not consistent with disk rotation and traces a gaseous outflow. We derived an outflow mass M out = 3.5 × 10 8 M , and a mass outflow rate in the range Ṁout = 1800 − 4500 M yr −1 . The implied Ėout ∼ 0.0005−0.001 L bol is in agreement with the values observed for ionised winds. For the first time, we mapped a spatially resolved water vapour disk through the H 2 O v=0 3 (2,2) − 3 (1,3) emission line detected at ν obs = 274.074 GHz, whose kinematic properties and size are broadly consistent with those of the [CII] disk. The luminosity ratio L H 2 O /L TIR = 1.4 × 10 −5 is consistent with line excitation by dust-reprocessed star formation in the interstellar medium of the host galaxy.

We present ALMA Band 9 continuum observation of the ultraluminous quasi-stellar object (QSO) SDSS J0100+2802 providing a ∼10σ detection at ∼670 GHz. SDSS J0100+2802 is the brightest QSO with the most massive supermassive black hole (SMBH) known at z > 6, and we study its dust spectral energy distribution in order to determine the dust properties and the star formation rate (SFR) of its host galaxy. We obtain the most accurate estimate so far of the temperature, mass, and emissivity index of the dust, which are T dust = 48.4 ± 2.3 K, M dust = (2.29 ± 0.83) × 107 M ⊙, and β = 2.63 ± 0.23, respectively. This allows us to measure the SFR with the smallest statistical error for this QSO, SFR = 265 ± 32 M ⊙yr−1. Our results enable us to evaluate the relative growth of the SMBH and host galaxy of J0100+2802. We find that the SMBH is dominating the process of black-hole galaxy growth in this QSO at z = 6.327, when the universe was 865 Myr old. Such unprecedented constraints on the host-galaxy SFR and dust temperature can only be obtained through high-frequency observations and highlight the importance of ALMA Band 9 to obtain a robust overview of the buildup of the first quasars’ host galaxies at z > 6.

We present a dynamical analysis of a quasar-host galaxy at z 6 (SDSS J2310+1855) using a high-resolution ALMA observation of the [CII] emission line. The observed rotation curve was fitted with mass models that considered the gravitational contribution of a thick gas disc, a thick star-forming stellar disc, and a central mass concentration, which is likely due to a combination of a spheroidal component (i.e. a stellar bulge) and a supermassive black hole (SMBH). The SMBH mass of 5 × 10 9 M , previously measured using the CIV and MgII emission lines, is not sufficient to explain the high velocities in the central regions. Our dynamical model suggests the presence of a stellar bulge with a mass of M bulge ∼ 10 10 M in this object, when the Universe was younger than 1 Gyr. To finally be located on the local M SMBH − M bulge relation, the bulge mass should increase by a factor of ∼40 from z = 6 to 0, while the SMBH mass should grow by a factor of 4 at most. This points towards asynchronous galaxy-BH co-evolution. Imaging with the JWST will allow us to validate this scenario.