Internal Kinematics of Field Galaxies at Intermediate Redshifts

Abstract

Low-mass starbursting galaxies have been proposed as the explanation of the excess of faint galaxies observed at intermediate redshifts. If this hypothesis is correct, then intermediate redshift galaxies should rotate more slowly than nearby galaxies with the same rest-frame luminosity. We present the results of a survey of the internal kinematics of intermediate redshift (z = 0.25-0.45) field galaxies to search for this effect. Using the Canada-France-Hawaii Telescope, spatially-resolved spectra of the [O II] $\lambda\lambda$ 3726- 3729 \AA doublet emission line have been obtained for 22 galaxies. V$_{\rm rot} $ sin i and [O II] disk scale lengths have been extracted from each galaxy spectrum using a Bayesian fitting technique. Galaxies in the sample are found to be $\sim$1.5-2.0 mag brighter than expected from their rotation velocity and the local Tully-Fisher (TF) relation. Low-mass galaxies exhibit a wider range of evolution relative to the TF relation than high-mass galaxies. The main source of uncertainty in this result is the large scatter in the local TF relation for late-type galaxies. Luminosity-dependent luminosity evolution neatly reconciles the lack of evolution seen in other works with the results of our survey. It is also found that the overall properties of [OII] kinematics at intermediate redshifts are varied. For example, 25% of the field galaxies in the sample have [OII] kinematics unrelated to rotation; [OII] emission is confined to the nucleus in most of these galaxies. Anomalous kinematics is found to be related to the presence of companions -- i.e. minor merger events. A Doppler ellipse similar to those found in local dwarf galaxies has been observed in a z = 0.35 galaxy, and may be interpreted as a supernova-driven supershell.