The Tully-Fisher Relation at Intermediate Redshifts

Abstract

Motivated by recent pioneering measurements of galaxy kinematics at intermediate redshifts, we have begun a pilot survey to measure rotation curves for blue, star-forming galaxies between 0.05<z<0.35. The scientific impetus for such measurements is to construct internal velocity - luminosity relations at redshifts substantial enough to make cosmological and evolutionary tests. Departures from a fiducial relation as a function of redshift are sensitive to both space-time curvature and the evolution of the mass-to-light ratio in galaxies. Because of this ambiguity -- curvature vs. evolution -- kinematics should be viewed as a new cosmological tool at intermediate redshifts to be combined with additional measurements. In particular, the apparent internal velocities, colors, surface-brightness, and image shape are all unaffected by curvature, so that this ensemble of observables can be used unambiguously to explore galaxy evolution. Here we discuss a well-defined method of target selection which assures efficient measurement of spatially and spectrally resolved kinematics, namely rotation curves. Spatial resolution is critical since it is difficult to use integrated line-widths to distinguish between, for example, a low-mass star-forming galaxy and a high-mass galaxy with a central star-burst. For 18 of 19 appropriately selected galaxies to B<20.5 from Bershady et al (1994), we have successfully measured rotation curves using the KAST spectrograph on the Lick 3m telescope with 1-2 hour integrations for each target. As a sanity check, we present preliminary results for half of our sample: We measure H0 to be ~75 km/s/Mpc at a median redshift of 0.15.