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

摘要

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 g r i z y photometry. Methods. We used the K -band Multi-Object Spectrograph (KMOS) to carry out H α integral field spectroscopy of 46 galaxies in total. Ionized gas maps, star formation rates, and velocity fields were derived from the H α 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. H α 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 V _(max)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 V _(max)and gas velocity dispersion σ _(g). This probably reflects that fewer disks have settled to purely rotational kinematics and high V _(max)/ σ _(g)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 ( σ _( v )?< ?400 km s~(?1)) of the (proto-)clusters, gravitational interactions are likely more efficient, resulting in higher kinematical asymmetries than in present-days clusters.