Radio sources at the highest redshifts can provide unique information on the first massive galaxies and black holes, the densest primordial environments, and the epoch of reionization. The number of astronomical objects identified at z > 6 has increased dramatically over the last few years, but previously only three radio-loud (R 2500 = f ν,5 GHz/f ν,2500 Å > 10) sources had been reported at z > 6, with the most distant being a quasar at z = 6.18. Here we present the discovery and characterization of PSO J172.3556+18.7734, a radio-loud quasar at z = 6.823. This source has an Mg ii-based black hole mass of ∼3 × 108 M ⊙ and is one of the fastest accreting quasars, consistent with super-Eddington accretion. The ionized region around the quasar is among the largest measured at these redshifts, implying an active phase longer than the average lifetime of the z ≳ 6 quasar population. From archival data, there is evidence that its 1.4 GHz emission has decreased by a factor of two over the last two decades. The quasar’s radio spectrum between 1.4 and 3.0 GHz is steep (α = −1.31). Assuming the measured radio slope and extrapolating to rest-frame 5 GHz, the quasar has a radio-loudness parameter R 2500 ∼ 90. A second steep radio source (α = −0.83) of comparable brightness to the quasar is only 23.″1 away (∼120 kpc at z = 6.82; projection probability <2%), but shows no optical or near-infrared counterpart. Further follow-up is required to establish whether these two sources are physically associated.

Context. The existence of tight correlations between supermassive black holes (BHs) and their host galaxies properties in the local Universe suggests a closely linked evolution. Investigating these relations up to the high redshifts (z 6) is crucial to understand the interplay between star-formation and BH growth across the cosmic time and to set constraints on galaxy formation and evolution models. In this work we focus on the relation between BH mass (M BH ) and the dynamical mass (M dyn ) of the host galaxy. Aims. Previous works suggest an evolution of the M BH − M dyn relation with redshift indicating that BH growth precedes the galaxy mass assembly during their co-evolution at z > 3. However, dynamical galaxy masses at high redshift are often estimated through the virial theorem thus introducing significant uncertainties. This work aims at studying the M BH − M dyn relation of a sample of 2 < z < 7 quasars by constraining their galaxy masses through a full kinematical modelling of the cold gas kinematics thus avoiding all possible biases and effects introduced by the rough estimates usually adopted so far. Methods. For this purpose we retrieved public observations of 72 quasar host galaxies observed in [CII] 158µm or CO transitions with the Atacama Large Millimeter Array (ALMA). We then selected those quasars whose line emission is spatially resolved and performed a kinematic analysis on ALMA observations. We estimated the dynamical mass of the systems by modelling the gas kinematics with a rotating disc taking into account geometrical and instrumental effects. Our dynamical mass estimates, combined with M BH obtained from literature and our own new CIVλ1550 observations have allowed us to investigate the M BH /M dyn in the early Universe. Results. Overall we obtained a sample of 10 quasars at z ∼ 2 − 7 in which line emission is detected with high SNR ( 5 − 10) and the gas kinematics is spatially resolved and dominated by ordered rotation. The estimated dynamical masses place 6 out of 10 quasars above the local relation yielding to a M BH /M dyn ratios ∼ 10× higher than those estimated in low-z galaxies. On the other hand we found that 4 quasars at z ∼ 4 − 6 have dynamical-to-BH mass ratios consistent with what is observed in early type galaxies in the local Universe.

We report new Northern Extended Millimeter Array observations of the [C ii]158 μm, [N ii]205 μm, and [O i]146 μm atomic fine structure lines (FSLs) and dust continuum emission of J1148+5251, a z = 6.42 quasar, which probe the physical properties of its interstellar medium (ISM). The radially averaged [C ii]158 μm and dust continuum emission have similar extensions (up to θ = 2.51 − 0.25 + 0.46 arcsec , corresponding to r = 9.8 − 2.1 + 3.3 kpc , accounting for beam convolution), confirming that J1148+5251 is the quasar with the largest [C ii]158 μm-emitting reservoir known at these epochs. Moreover, if the [C ii]158 μm emission is examined only along its NE–SW axis, a significant excess (>5.8σ) of [C ii]158 μm emission (with respect to the dust) is detected. The new wide-bandwidth observations enable us to accurately constrain the continuum emission, and do not statistically require the presence of broad [C ii]158 μm line wings that were reported in previous studies. We also report the first detection of the [O i]146 μm and (tentatively) [N ii]205 μm emission lines in J1148+5251. Using FSL ratios of the [C ii]158 μm, [N ii]205 μm, [O i]146 μm, and previously measured [C i]369 μm emission lines, we show that J1148+5251 has similar ISM conditions compared to lower-redshift (ultra)luminous infrared galaxies. CLOUDY modeling of the FSL ratios excludes X-ray-dominated regions and favors photodissociation regions as the origin of the FSL emission. We find that a high radiation field (103.5–4.5 G 0), a high gas density (n ≃ 103.5–4.5 cm−3), and an H i column density of 1023 cm−2 reproduce the observed FSL ratios well.

We present a multiline survey of the interstellar medium (ISM) in two z > 6 quasar host galaxies, PJ231−20 (z = 6.59) and PJ308−21 (z = 6.23), and their two companion galaxies. Observations were carried out using the Atacama Large (sub-)Millimeter Array (ALMA). We targeted 11 transitions including atomic fine-structure lines (FSLs) and molecular lines: [NII]205 μm, [CI]369 μm, CO (Jup = 7, 10, 15, 16), H2O 312 − 221, 321 − 312, 303 − 212, and the OH163 μm doublet. The underlying far-infrared (FIR) continuum samples the Rayleigh-Jeans tail of the respective dust emission. By combining this information with our earlier ALMA [CII]158 μm observations, we explored the effects of star formation and black hole feedback on the ISM of the galaxies using the CLOUDY radiative transfer models. We estimated dust masses, spectral indexes, IR luminosities, and star-formation rates from the FIR continuum. The analysis of the FSLs indicates that the [CII]158 μm and [CI]369 μm emission arises predominantly from the neutral medium in photodissociation regions (PDRs). We find that line deficits agree with those of local luminous IR galaxies. The CO spectral line energy distributions (SLEDs) reveal significant high-J CO excitation in both quasar hosts. Our CO SLED modeling of the quasar PJ231−20 shows that PDRs dominate the molecular mass and CO luminosities for Jup ≤ 7, while the Jup ≥ 10 CO emission is likely driven by X-ray dissociation regions produced by the active galactic nucleus (AGN) at the very center of the quasar host. The Jup > 10 lines are undetected in the other galaxies in our study. The H2O 321 − 312 line detection in the same quasar places this object on the LH2O − LTIR relation found for low-z sources, thus suggesting that this water vapor transition is predominantly excited by IR pumping. Models of the H2O SLED and of the H2O-to-OH163 μm ratio point to PDR contributions with high volume and column density (nH ∼ 0.8 × 105 cm−3, NH = 1024 cm−2) in an intense radiation field. Our analysis suggests a less highly excited medium in the companion galaxies. However, the current data do not allow us to definitively rule out an AGN in these sources, as suggested by previous studies of the same objects. This work demonstrates the power of multiline studies of FIR diagnostics in order to dissect the physical conditions in the first massive galaxies emerging from cosmic dawn.