Autofib Redshift Survey - I. Evolution of the galaxy luminosity function

We present a detailed determination of the rest frame B-band galaxy luminosity function (LF) as a function of redshift and star formation activity from z = 0 to 2 < 0.75. The data set used for this purpose is a composite sample of over 1700 redshifts spanning a wide range in apparent magnitude, 11.5 < bJ < 24.0, which we term the Autofib Redshift Survey. The sample includes various earlier magnitude-limited surveys constructed by our team, as well as a new survey of 1026 redshifts measured for galaxies at intermediate magnitudes. Spectral classifications, essential for estimating the k-corrections and galaxy luminosities, are accomplished via cross-correlation with Kennicutt's library of integrated galaxy spectra. The various overlapping surveys in the sample enable us to assess the effects of redshift incompleteness. We demonstrate that uncertainties in classification and those arising from incompleteness do not seriously affect our conclusions. The large range in apparent magnitude sampled allows us to investigate both the nature of the LF at low redshifts (z < 0.1) and the possible evolution in its shape to z = 0.75. We find that earlier bright surveys have underestimated the absolute normalization of the LF. Because the shape of the local LF does not change with the apparent-magnitude limit of the survey, it seems unlikely that the local deficiency arises from an underestimated population of low-luminosity galaxies. Furthermore, surface-brightness losses cannot be significant unless they conspire to retain the LF shape over a variety of detection thresholds. Our data directly demonstrate that the B-band LF evolves with redshift. This evolution is best represented as a steepening of the faint-end slope of the LF, from α < -1.1 at low redshift to α < -1.5 at z < 0.5. Using [O II] emission as an indicator of star-formation activity, we show that the LF of quiescent galaxies has remained largely unchanged since z < 0.5, whereas the luminosity density of star-forming galaxies has declined by more than 50 per cent. The steepening of the overall LF with look-back time is of the form originally postulated by Broadhurst, Ellis & Shanks and is a direct consequence of the increasing space density of blue star-forming galaxies at moderate redshifts.