The Durham/Anglo–Australian Telescope faint galaxy redshift survey

Abstract

Using the fibre optic coupler at the Anglo–Australian Observatory, we have completed a new faint galaxy redshift survey. Intermediate dispersion spectra with resolution ∼4 Å have been gathered for over 200 field galaxies selected in apparent magnitude slices between $$20.0\lt b_J\lt21.5$$ mag in five high-latitude fields. Redshift completeness is 85 per cent and the mean redshift agrees well from field to field. Although the slope of the number–magnitude–count relation is considerably steeper than no-evolution predictions at $$b_J\sim21$$ mag, the redshift range observed is similar to that expected in a non-evolving population. If our fields are representative, luminosity evolution can only be occurring in low-luminosity galaxies over the past 5 Gyr (H₀ = 50 km s⁻¹ Mpc⁻¹ used here and throughout the paper). However, the faint spectra reveal strong evidence for enhanced star formation in a large proportion of the galaxies beyond z ∼ 0.1, which cannot be explained in terms of colour or aperture selection-effects. We suggest that it is these star-forming galaxies which represent the excess in the galaxy counts, at least at $$b_J\sim 21-22$$ mag. The colour and absorption-line features in these galaxies can only be reproduced with models incorporating strong short-lived bursts of star formation which temporarily brighten these otherwise low-luminosity systems. We outline a physical model accounting for the observations whereby only galaxies at the faint end of the luminosity function evolve via discrete short-lived bursts of star formation.