We present a study of the [C ii] 158 μm line and underlying far-infrared (FIR) continuum emission of 27 quasar host galaxies at z ∼ 6, traced by the Atacama Large Millimeter/submillimeter Array at a spatial resolution of ∼1 physical kpc. The [C ii] emission in the bright, central regions of the quasars have sizes of 1.0–4.8 kpc. The dust continuum emission is typically more compact than [C ii]. We find that 13/27 quasars (approximately one-half) have companion galaxies in the field, at projected separations of 3–90 kpc. The position of dust emission and the Gaia-corrected positions of the central accreting black holes are cospatial (typical offsets ≲0.″1). This suggests that the central black holes are located at the bottom of the gravitational wells of the dark matter halos in which the quasar hosts reside. Some outliers with offsets of ∼500 pc can be linked to disturbed morphologies, most likely due to ongoing or recent mergers. We find no correlation between the central brightness of the FIR emission and the bolometric luminosity of the accreting black hole. The FIR-derived star formation rate densities (SFRDs) in the host galaxies peak at the galaxies’ centers, at typical values between 100 and 1000 M ⊙ yr−1 kpc−2. These values are below the Eddington limit for star formation, but similar to those found in local ultraluminous infrared galaxies. The SFRDs drop toward larger radii by an order of magnitude. Likewise, the [C ii]/FIR luminosity ratios of the quasar hosts are lowest in their centers (few ×10−4) and increase by a factor of a few toward the galaxies’ outskirts, consistent with resolved studies of lower-redshift sources.

We investigate the CO excitation and interstellar medium (ISM) conditions in a cold gas mass-selected sample of 22 star-forming galaxies at z=0.46-3.60, observed as part of the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). Combined with Very Large Array follow-up observations, we detect a total of 34 CO -J J 1 transitions with J=1 up to 8 (and an additional 21 upper limits, up to J=10) and 6 [ ] C I  P P 10 5 , comparable to the Milky Way and main-sequence galaxies at similar redshifts, and fairly high densities (10 4 cm −3), consistent with the low-J CO excitation. Our results imply a decrease in the cosmic molecular gas mass density at z2 compared to previous ASPECS measurements.

We explore the kinematics of 27 z ≳ 6 quasar host galaxies observed in [C ii] 158 μm ([C ii]) emission with the Atacama Large Millimeter/submillimeter Array at a resolution of ≈025. We find that nine of the galaxies show disturbed [C ii] emission, due to either a close companion galaxy or a recent merger. Ten galaxies have smooth velocity gradients consistent with the emission arising from a gaseous disk. The remaining eight quasar host galaxies show no velocity gradient, suggesting that the gas in these systems is dispersion dominated. All galaxies show high velocity dispersions with a mean of 129 ± 10 km s−1. To provide an estimate of the dynamical mass within twice the half-light radius of the quasar host galaxy, we model the kinematics of the [C ii] emission line using our publicly available kinematic fitting code, qubefit. This results in a mean dynamical mass of 5.0 ± 0.8( ± 3.5) × 1010 M ⊙. Comparison between the dynamical mass and the mass of the supermassive black hole reveals that the sample falls above the locally derived bulge mass–black hole mass relation at 2.4σ significance. This result is robust even if we account for the large systematic uncertainties. Using several different estimators for the molecular mass, we estimate a gas mass fraction of >10%, indicating that gas makes up a large fraction of the baryonic mass of z ≳ 6 quasar host galaxies. Finally, we speculate that the large variety in [C ii] kinematics is an indication that gas accretion onto z ≳ 6 supermassive black holes is not caused by a single precipitating factor.

Using the NSF's Karl G. Jansky Very Large Array (VLA), we report six detections of CO(J=1 → 0) emission and one upper limit in z=2-3 galaxies originally detected in higher-J CO emission in the Atacama Large Millimeter/submillimeter Array Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). From the CO(J=1 → 0) line strengths, we measure total cold molecular gas masses of M gas =(2.4-11.6)×10 10 (α CO /3.6)M e. We also measure a median CO(J=3 → 2) to CO(J=1 → 0) line brightness temperature ratio of r 31 =0.84±0.26, and a CO(J=7 → 6) to CO(J=1 → 0) ratio range of r 71 <0.05 to r 71 =0.17. These results suggest that CO(J=3 → 2) selected galaxies may have a higher CO line excitation on average than CO(J=1 → 0) selected galaxies, based on the limited, currently available samples from the ASPECS and VLA CO Luminosity Density at High Redshift (COLDz) surveys. This implies that previous estimates of the cosmic density of cold gas in galaxies based on CO(J=3 → 2) measurements should be revised down by a factor of ;2 on average based on assumptions regarding CO excitation alone. This correction further improves the agreement between the best currently existing constraints on the cold gas density evolution across cosmic history from line scan surveys, and the implied characteristic gas depletion times.