The internal kinematics of intermediate redshift galaxies




Simard, Luc

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A dilemma is posed by studies of galaxy evolution at intermediate redshifts. If evolutionary effects are neglected, simple models predict number densities of faint galaxies which are 2—5x lower than observed at z ≅ 0.4. Yet the faint galaxy redshift distribution appears to be well modelled by the same no—evolution models. If low-mass starbursting galaxies are responsible for the excess, then the excess faint galaxy population should have rotation velocities lower than those of quiescent galaxies with the same luminosity. This thesis describes the results of a limited survey of the internal kinematics of intermediate redshift (z = 0.25—0.45) field galaxies. The goal of this survey was to find the unmistakable kinematical signature of low-mass starbursting galaxies. Using the Canada-France-Hawaii Telescope, spatially resolved spectra of the [O II] λλ 3726—3729 Å doublet emission line have been obtained for 22 galaxies. High-spatial resolution has made it possible to extract [special characters omitted] and [O II] disk scale length from each galaxy spectrum using synthetic galaxy rotation curve fitting. It is found that about 25% of the galaxies in the sample have [OIl] kinematics unrelated to rotation. [OIl] emission is concentrated in the nucleus in these “kinematically anomalous" galaxies. A Doppler ellipse similar to those found in local dwarf irregular galaxies has been observed in a z = 0.35 galaxy. An intermediate redshift Tully-Fisher (TF) relation defined by 12 kinematically normal galaxies shows that these galaxies have a systematically lower rotation velocity (i.e. mass) for their luminosity than expected from the local TF relation. These galaxies would have to fade by ~ 1.5-2 mag to lie on the local TF relation. This is consistent with starbursting dwarf galeixy models. Although the sample is small, there is also a hint that massive galaxies do not lie as far off the local TF relation as low-mass ones. However, as shown using a large sample of local galaxies, the scatter in the local TF relation is large, especially for late-type galaxies. Selection effects, particularly [OIl] emission strength, could be responsible for part of the observed TF shift if different star formation rates are responsible for the local TF scatter. A comparison with other works indicates that the luminosity-dependent luminosity evolution scenario neatly explains all the available internal kinematics and surface brightness data.



Kinematics, Red shift, Galaxies