Internal kinematics of distant field galaxies .1. Emission linewidths for a complete sample of faint blue galaxies at (z)similar to 0.25

We present measurements of the [O II] emission linewidth for a complete sample of 24 blue field galaxies (21.25 < B < 22, B - R < 1.2) at [z] similar to 0.25, obtained with the AUTOFIB fibre spectrograph on the Angle-Australian Telescope (AAT). Most emission lines are spectrally resolved, yet all have dispersions sigma(upsilon) < 100 km s(-1). Five of the 24 sample members have [O II] 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 19 linewidths are too large by a factor of 2 (7 sigma 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 similar to 0.25; these allow us to compute the expected distribution of (observable) linewidths sigma(upsilon) for a galaxy of a given circular rotation speed upsilon(c). 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 log[upsilon(c)(M(B),B-R)] = log [upsilon(c)(-19, 1)] - eta (M(B)+19) + zeta[(B - R) - 1] and determine the range of parameters consistent with our data. We find a mean rotation speed of upsilon(c)(-19, 1) = 66+/-8km s(-1) (68 per cent confidence limits) for distant galaxies with Mg = -19 and B - R = 1, with a magnitude dependence for upsilon(c) of eta = 0.07+/-0.08, and a colour dependence of zeta = 0.28+/-0.25. Through comparison with several local galaxy samples, we show that this value of upsilon(c)(-19, 1) is significantly lower than the optical rotation speed of present-day galaxies with the same absolute magnitude and rest-frame colour (approximate to 105 km s(-1)). The most straightforward interpretation is that the distant blue, sub-L* galaxies are about 1.5 mag brighter (and greater than or equal to 0.8 mag brighter at 99 per cent confidence) than local galaxies of the same linewidth and colour.