The LDSS deep redshift survey.

Using a new multislit spectrograph, LDSS, we have obtained intermediate dispersion spectroscopy for a new sample of 149 faint objects selected randomly from the magnitude range 21 <= b_J_ <= 22.5 in three high-latitude fields. Our overall success rate of 81 per cent for spectroscopic identification is comparable to that achieved in the earlier fibre survey of Broadhurst et al. to b_J_ = 21.5, demonstrating that LDSS has pushed the limiting magnitude for field surveys 1 mag deeper. Of identified objects, 25 per cent are galactic stars, in reasonable agreement with estimates based on image classification. The remaining objects are galaxies with redshifts 0 < z < 0.7. Despite the 19 per cent incompleteness, the extended wavelength coverage and continuum signal-to-noise ratio of our spectra imply that the observed redshift distribution is representative of galaxies at these magnitudes, and that we can rule out a significant excess of high- redshift galaxies. At b_J_ = 22.5, the galaxy counts are approximately twice the no-evolution model prediction, yet we find the redshift distribution to be close to that expected for a non-evolving population. At least half the excess population must therefore have z < 0.5. The absence of any identified galaxies with z > 0.7 to b_J_ = 22.5 provides important constraints on models where galaxy luminosities evolve monotonically with redshift in a luminosity-independent way. In particular, we can place new limits on the evolution of luminous galaxies since z ~1.5-2. Our data support earlier suggestions that the steep slope of the number-magnitude counts is mostly due to the evolution of galaxies at the faint end of the galaxy luminosity function. We also find a significant excess over non- evolving models in the proportion of galaxies with strong [O II] 3727 A emission, indicating that star formation is more common at these faint magnitudes. The implications of our spectra for a proposed population of high-redshift flat-spectrum galaxies with high star-formation rates are also discussed.