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 OII (3727) emission line width for a complete sample of 24 blue field galaxies (21. 25=0. 25, obtained with the AUTOFIB fibre spectrograph on the Anglo-Australian Telescope. Most emission lines are spectrally resolved, yet all have dispersions sigma<100km/s. Five of the 24 sample members have OII doublet line flux ratios which 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 19 linewidths are too large by a factor of two (7sigma 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 datasets that mimic our observational setup at z=0. 25; these allow us to compute the expected distribution of (observable) linewidths sigmaᵥ for a galaxy of a given ``true'' (optical) rotation speed vc. These simulations include the effects of random viewing angles, clumpy line emission, finite fibre aperture, and internal dust extinction on the emission line profile. We assume a linewidth--luminosity--colour relation: ln vc (MB, B-R) = lnvc (-19, 1) - eta* (MB+10) + zeta* (B-R) -1 and determine the range of parameters consistent with our data. We find a mean rotation speed of vc (-19, 1) =66+-8km/s (68% confidence limits) for the distant galaxies with MB=-19 and B-R=1, with a magnitude dependence for vc of eta=0. 07+-0. 08, and a colour dependence of zeta =0. 28+-0. 25. Through comparison with several local 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