2020MNRAS.499.1250K

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 µm 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 (≃30 pc), the luminosity-weighted average velocity dispersion is σ_ CII_≃23-38 km s^–1 with the highest value belonging to the highly structured Disturbed Disc stage. Low-resolution observations tend to overestimate σ_C 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 per cent (52-115 per cent) larger than the actual one. The [C II] emitting gas in Freesia has a Toomre parameter Q≃0.2 and rotational-to-dispersion ratio of v_c/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 (< 10 per cent). Finally, we find that the resolved σ_C II_–ΣSFR_ relation is relatively flat for 0.02< Σ_ SFR_/ M_☉ yr^–1 kpc^–2 < 30, with the majority of data lying on the derived analytical relation σ ∝Σ_ SFR_^5/7. At high SFR, the increased contribution from stellar feedback steepens the relation, and σ_C II_ rises slightly.