The SINS/zC-SINF Survey of z~2 Galaxy Kinematics: The Nature of Dispersion Dominated Galaxies

TL;DR

This study investigates the kinematic properties of high-redshift star-forming galaxies, revealing that smaller, more compact galaxies tend to be dispersion-dominated due to size-related effects and beam smearing, and may represent an earlier evolutionary stage.

Contribution

It provides a detailed analysis of the relationship between galaxy size and kinematic classification at z~2, highlighting the role of beam smearing and intrinsic properties in dispersion dominance.

Findings

Smaller galaxies are more likely dispersion-dominated.

Kinematic classification correlates with galaxy size.

Dispersion-dominated galaxies may be earlier evolutionary stages.

Abstract

We analyze the spectra, spatial distributions and kinematics of Ha, [NII] and [SII] emission in a sample of 42, z~2.2 UV/optically selected star forming galaxies (SFGs) from the SINS & zC-SINF surveys, 35 of which were observed in the adaptive optics mode of SINFONI. This is supplemented by kinematic data from 48 z~1-2.5 galaxies from the literature. We find that the kinematic classification of the high-z SFGs as `dispersion dominated' or `rotation dominated' correlates most strongly with their intrinsic sizes. Smaller galaxies are more likely `dispersion-dominated' for two main reasons: 1) The rotation velocity scales linearly with galaxy size but intrinsic velocity dispersion does not depend on size, and as such, their ratio is systematically lower for smaller galaxies, and 2) Beam smearing strongly decreases large-scale velocity gradients and increases observed dispersion much more for galaxies with sizes at or below the resolution. Dispersion dominated SFGs may thus have intrinsic properties similar to `rotation dominated' SFGs, but are primarily more compact, lower mass, less metal enriched and may have higher gas fractions, plausibly because they represent an earlier evolutionary state.