The Kinematics of z ≳ 6 Quasar Host Galaxies

2021 
We explore the kinematics of 27 z~6 quasar host galaxies observed in [CII]-158 micron ([CII]) emission with the Atacama Large Millimeter/sub-millimeter Array at a resolution of ~0.25''. We find that nine of the galaxies show disturbed [CII] emission, either due to a close companion galaxy or 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. 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 [CII] emission line using our publicly available kinematic fitting code, qubefit. This results in a mean dynamical mass of 5.0+-0.8(+-3.5) x 10^10 Msun. 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 2.4sigma 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 gas makes up a large fraction of the baryonic mass of z~6 quasar host galaxies. Finally, we speculate that the large variety in [CII] kinematics is an indication that gas accretion onto z~6 super massive black holes is not caused by a single precipitating factor.
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