The KMOS Deep Survey (KDS) I: dynamical measurements of typical star-forming galaxies at z $\simeq$ 3.5

Abstract

We present dynamical measurements from the KMOS (K-band Multi-Object Spectrograph) Deep Survey (KDS), which is comprised of 77 typical star-forming galaxies at z $\simeq$ 3.5 in the mass range 9.0 < log(M$_{\star}$/M$_{\odot}$) < 10.5. These measurements constrain the internal dynamics, the intrinsic velocity dispersions (σ$_{int}$) and rotation velocities (V$_{C}$) of galaxies in the high redshift Universe. The mean velocity dispersion of the galaxies in our sample is σ$_{int}$ = $70.8^{+3.3}_{-3.1}$ km s$^{-1}$, revealing that the increasing average σ$_{int}$ with increasing redshift, reported for z $\lesssim2$, continues out to z $\simeq$ 3.5. Only 34 $\pm$ 8% of our galaxies are rotation-dominated (V$_{C}$/σ$_{int}$ > 1), with the sample average V$_{C}$/σ$_{int}$ value much smaller than at lower redshift. After carefully selecting comparable star-forming samples at multiple epochs, we find that the rotation-dominated fraction evolves with redshift with a z$^{-0.2}$ dependence. The rotation-dominated KDS galaxies show no clear offset from the local rotation velocity-stellar mass (i.e. V$_{C}$-M$_{\star}$) relation, although a smaller fraction of the galaxies are on the relation due to the increase in the dispersion-dominated fraction. These observations are consistent with a simple equilibrium model picture, in which random motions are boosted in high redshift galaxies by a combination of the increasing gas fractions, accretion efficiency, specific star-formation rate and stellar feedback and which may provide significant pressure support against gravity on the galactic disk scale.

main body 25 pages, 8 figures