Velocity dispersion in the interstellar medium of early galaxies

Abstract

ABSTRACT We study the structure of spatially resolved, line-of-sight velocity dispersion for galaxies in the Epoch of Reionization (EoR) traced by [C $\scriptstyle \rm II$] $158\, \mu \rm {m}$ line emission. Our laboratory is a simulated prototypical Lyman-break galaxy, ‘Freesia, part of the serra suite’. The analysis encompasses the redshift range 6 < z < 8, when Freesia is in a very active assembling phase. We build velocity dispersion maps for three dynamically distinct evolutionary stages (Spiral Disc at z = 7.4, Merger at z = 8.0, and Disturbed Disc at z = 6.5) using [C $\scriptstyle \rm II$] hyperspectral data cubes. We find that, at a high spatial resolution of 0.005 arcsec (≃30 pc), the luminosity-weighted average velocity dispersion is $\sigma _{\rm {CII}}\simeq 23{\text {--}}38\, {\rm km\, s^{-1}}$ with the highest value belonging to the highly structured Disturbed Disc stage. Low-resolution observations tend to overestimate σC ii values due to beam smearing effects that depend on the specific galaxy structure. For an angular resolution of 0.02 arcsec (0.1 arcsec), the average velocity dispersion is $16{\!-\!}34{{\ \rm per\ cent}}$ ($52{\!-\!}115{{\ \rm per\ cent}}$) larger than the actual one. The [C $\scriptstyle \rm II$] emitting gas in Freesia has a Toomre parameter $\mathcal {Q}\simeq 0.2$ and rotational-to-dispersion ratio of vc/σ ≃ 7 similar to that observed in z = 2−3 galaxies. The primary energy source for the velocity dispersion is due to gravitational processes, such as merging/accretion events; energy input from stellar feedback is generally sub-dominant ($\lt 10{{\ \rm per\ cent}}$). Finally, we find that the resolved σC ii−ΣSFR relation is relatively flat for $0.02\lt {\Sigma }_{\rm SFR}/{\rm M}_{\odot }\rm {yr}^{-1} {\rm kpc}^{-2} \lt 30$, with the majority of data lying on the derived analytical relation $\sigma \propto \Sigma _{\rm SFR}^{5/7}$. At high SFR, the increased contribution from stellar feedback steepens the relation, and σC ii rises slightly.