The Kinematics of z ≳ 6 Quasar Host Galaxies

Abstract

We explore the kinematics of 27 z ≳ 6 quasar host galaxies observed in [C ii ] 158 μ m ([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) × 10 10 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 2.4 σ 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.