The KMOS Deep Survey (KDS) - I. Dynamical measurements of typical star-forming galaxies at z ≃ 3.5

Abstract

We present dynamical measurements from the KMOS (K-band multi-object spectrograph) Deep Survey (KDS), which comprises 77 typical star-forming galaxies at z ≃ 3.5 in the mass range 9.0 < log (M_⋆/M_⊙) < 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 ≲ 2, continues out to z ≃ 3.5. Only 36 ± 8 per cent 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_⋆) 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 disc scale.