Evolution of Gas Velocity Dispersion in Discs from $z\sim8$ to $z\sim0.5$

Abstract

Together optical/near infrared integral field spectroscopy and resolved sub-millimetre interferometry data have mapped the ionised and molecular gas motions in nearly one thousand galaxies at redshifts $z>0.5$. While these measurements have revealed a number of key properties about the evolution of disc structure and kinematics, heterogenous techniques and samples have led to disparate findings - especially when comparing different dynamical tracers (e.g., H$\alpha$, [C$\scriptstyle\rm~II$], CO). In this paper we present a literature compilation of 237 disc galaxies with measurements of velocity dispersion and rotational velocity between $z=0.5-8$, a subset of 63 galaxies have measurements of molecular gas fractions. We explore the connection between disc velocity dispersion measurements over 8 Gyrs as traced by multiple phases with the expectations from Toomre stability models. When sample properties are taken into account (e.g., stellar mass, tracer) there is little evolution in disc dispersions between $z\sim1.5-8$, consistent with expectations from model assumptions. We find ionised gas dispersions are higher by $\sim2\times$ from molecular gas dispersions at a fixed gas mass. These results are sensitive to the molecular gas tracer with results from [C$\scriptstyle\rm~II$] showing mixed behaviour indicative of its multi-phase origin. The [C$\scriptstyle\rm~II$] kinematics can be reconciled with molecular and ionised gas tracers when star-formation rates are taken into account.