INTERNAL KINEMATICS OF DISTANT FIELD GALAXIES .1. EMISSION LINEWIDTHSFOR A COMPLETE SAMPLE OF FAINT BLUE GALAXIES AT (Z)SIMILAR-TO-0.25

We present measurements of the [O II] emission linewidth for a complet e 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 t hat 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 li newidths are too large by a factor of 2 (7 sigma significance) to be a ttributed to turbulent motions within an individual star-forming regio n, 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 o f (observable) linewidths sigma(upsilon) for a galaxy of a given circu lar rotation speed upsilon(c). These simulations include the effects o f random viewing angles, clumpy line emission, finite fibre aperture a nd internal dust extinction on the profile of the emission line. We as sume 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 det ermine the range of parameters consistent with our data. We find a mea n rotation speed of upsilon(c)(-19, 1) = 66+/-8km s(-1) (68 per cent c onfidence limits) for distant galaxies with Mg = -19 and B - R = 1, wi th a magnitude dependence for upsilon(c) of eta = 0.07+/-0.08, and a c olour dependence of zeta = 0.28+/-0.25. Through comparison with severa l 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 (appro ximate to 105 km s(-1)). The most straightforward interpretation is th at the distant blue, sub-L galaxies are about 1.5 mag brighter (and g reater than or equal to 0.8 mag brighter at 99 per cent confidence) th an local galaxies of the same linewidth and colour.