Keck spectroscopy and Hubble Space Telescope imaging of field galaxies at moderate redshift

We present 18 spectra, obtained with the Keck 10 m telescope, of faint field galaxies (19 < I < 22, 0.2 < z < 0.84) previously imaged by the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2 (WFPC2). Though small, our sample appears to be representative of field spirals with a magnitude limit of I less than or equal to 22. Combining the results from the spectral and imaging data, we have derived various quantitative parameters for the galaxies, including colors, inclinations, emission-line equivalent widths, redshifts, luminosities, internal velocity information, and physical scale lengths. In particular, disk scale lengths (with sizes ranging from similar to 1 to 5 kpc) have been measured from fits to the surface brightness profiles. We have also measured internal velocities with a rest frame resolution of sigma = 55-80 km s(-1) by fitting to the emission lines. The luminosity-disk size and luminosity-internal velocity (Tully-Fisher) relations for our moderate redshift galaxies are similar to the scaling relations seen for local galaxies, albeit with a modest brightening of similar to 1 mag. The one bulge-dominated galaxy in our sample (at z = 0.324) has a relatively blue color, reveals weak emission lines, and is similar to 0.5 mag brighter in the rest frame than expected for a passive local elliptical. Our data suggest that galaxies at about half the age of the universe have undergone mild luminosity evolution to the present epoch but are otherwise quantitatively similar to galaxies seen locally.