Internal kinematics of distant field galaxies - I. Emission linewidths for a complete sample of faint blue galaxies at ~0.25

We present measurements of the [0 II] emission linewidth for a complete sample of 24 blue field galaxies (21.25 < B < 22, B - R < 1.2) at (z) - 0.25, obtained with the AUTOFIB fibre spectrograph on the Anglo-Australian Telescope (AAT). Most emission lines are spectrally resolved, yet all have dispersions Uv < 100km S-l. Five of the 24 sample members have [On] doublet lines with flux ratios that imply gas densities in excess of 100 cm -3. The line emission in these galaxies may be dominated by an active nucleus and the galaxies have been eliminated from the subsequent analysis. The remaining 19linewidths are too large by a factor of 2 (7u significance) to be attributed to turbulent motions within an individual star-forming region, and therefore most likely reflect the orbital motion of ionized gas in the galaxy. We use Fabry­ Perot observations of nearby galaxies to construct simulated data sets that mimic our observational setup at z - 0.25; these allow us to compute the expected distribution of (observable) linewidths Uv for a galaxy of a given circular rotation speed Vc. These simulations include the effects of random viewing angles, clumpy line emission, finite fibre aperture and internal dust extinction on the profile of the emission line. We assume a linewidth­ luminosity -colour relation and determine the range of parameters consistent with our data. We find a mean rotation speed of vc(-19, 1) = 66±8kms-1 (68 per cent confidence limits) for distant galaxies with MB = -19 and B - R = 1, with a magnitude dependence for Vc of 7J = 0.07 ± 0.08, and a colour dependence of r = 0.28 ± 0.25. Through comparison with several local galaxy samples, we show that this value of vc( -19, 1) is significantly lower than the optical rotation speed of present-day galaxies with the same absolute magnitude and rest-frame colour ("" 1 05 km s -1). The most straightforward interpretation is that the distant blue, sub-L * galaxies are about 1.5 mag brighter (and ~0.8 mag brighter at 99 per cent confidence) than local galaxies of the same linewidth and colour.