The Kinematics of z ~ 6 Quasar Host Galaxies

TL;DR

This study investigates the kinematic properties of 27 z~6 quasar host galaxies using ALMA observations, revealing diverse gas dynamics, high velocity dispersions, and implications for galaxy-black hole mass relations and gas accretion processes.

Contribution

It provides the first comprehensive kinematic analysis of a sizable sample of z~6 quasar hosts, highlighting the diversity in gas dynamics and their relation to galaxy evolution.

Findings

Nine galaxies show disturbed [CII] emission due to mergers or companions.

Ten galaxies exhibit smooth velocity gradients consistent with gaseous disks.

The average dynamical mass within the host galaxies is approximately 5 x 10^10 solar masses.

Abstract

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.