Velocity dispersion in the interstellar medium of early galaxies

We study the structure of spatially resolved, line-of-sight velocity dispersion for galaxies in the Epoch of Reionization (EoR) traced by [C II] 158 mu 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 II] hyperspectral data cubes. We find that, at a high spatial resolution of 0.005 arcsec (similar or equal to 30 pc), the luminosity-weighted average velocity dispersion is sigma(CII) similar or equal to 23-38 km s(-1) with the highest value belonging to the highly structured Disturbed Disc stage. Low-resolution observations tend to overestimate sigma(CII) 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 per cent (52-115 per cent) larger than the actual one. The [C II] emitting gas in Freesia has a Toomre parameter Q similar or equal to 0.2 and rotational-to-dispersion ratio of v(c)/sigma similar or equal to 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 (< 10 per cent). Finally, we find that the resolved sigma(CII) - Sigma(SFR) relation is relatively flat for 0.02 < Sigma(SFR)/M-circle dot yr(-1)kpc(-2) < 30, with the majority of data lying on the derived analytical relation sigma proportional to Sigma(5/7)(SFR). At high SFR, the increased contribution from stellar feedback steepens the relation, and sigma(CII) rises slightly.