Kinematics of disk galaxies in (proto-)clusters at z = 1.5

Abstract

Aims. While many aspects of the impact of dense environments on late-type galaxies at redshifts below unity have been scrutinized in the past few decades, observational studies of the interplay between environment and disk galaxy evolution at z >  1 are still scarce. We observed star-forming galaxies at z ≈ 1.5 selected from the HyperSuprimeCam Subaru Strategic Program. The galaxies are part of two significant overdensities of [O II] emitters identified via narrowband imaging and photometric redshifts from grizy photometry. Methods. We used the K-band Multi-Object Spectrograph (KMOS) to carry out Hα integral field spectroscopy of 46 galaxies in total. Ionized gas maps, star formation rates, and velocity fields were derived from the Hα emission line. We quantified morphological and kinematical asymmetries in order to look for potential gravitational (e.g., galaxy-galaxy) or hydrodynamical (e.g., ram-pressure) interactions. Results. Hα emission was detected in 36 of our targets. Of these galaxies, 34 are members of two (proto-)clusters at z = 1.47, confirming our selection strategy to be highly efficient. By fitting model velocity fields to the observed ones, we determined the intrinsic maximum rotation velocity Vmax of 14 galaxies. Utilizing the luminosity–velocity (Tully–Fisher) relation, we find that these galaxies are more luminous than their local counterparts of similar mass by up to ∼4 mag in the rest-frame B-band. In contrast to field galaxies at z <  1, the offsets of the z ≈ 1.5 (proto-)cluster galaxies from the local Tully–Fisher relation are not correlated with their star formation rates but with the ratio between Vmax and gas velocity dispersion σg. This probably reflects that fewer disks have settled to purely rotational kinematics and high Vmax/σg ratios, as is observed in the field at similar redshifts. Tests with degraded low-redshift cluster galaxy data show that we cannot identify purely hydrodynamical interactions with the imaging currently at hand. Due to relatively low galaxy velocity dispersions (σv <  400 km s−1) of the (proto-)clusters, gravitational interactions are likely more efficient, resulting in higher kinematical asymmetries than in present-days clusters.