A faint galaxy redshift survey and implications for cosmology

Abstract

A new faint galaxy redshift survey has been constructed using the fibre optic coupler at the Anglo-Australian Observatory. Intermediate dispersion spectra with resolution ~ 4 A have been gathered for over 200 field galaxies selected in apparent magnitude slices between 20.0 < b(_j) < 21.5 in 5 high latitude fields. Redshift completeness is 85% and the mean redshift agrees well from field to field Models for the prediction of faint galaxy distributions are constructed from a bright galaxy redshift survey (Durham/Anglo-Australian Redshift Survey) and show that although the slope of the galaxy number-magnitude count relation is considerably steeper than no-evolution predictions at b(_j) ~ 21, the redshift range observed is similar to that expected for a non-evolving population. If our fields are representative, luminosity evolution can only be occurring in low luminosity galaxies to (_z) < 0.5.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 in the range b(_j) < 22.The colour and absorption line features of the stronger emission fine galaxies can only be reproduced with models incorporating strong short-lived bursts of star formation which temporarily brighten these otherwise low-luminosity systems. We are able to reproduce these observations with a physical 'burst' model in which only galaxies at the faint end of the luminosity function significantly evolve via short (lO(^8)yrs) bursts of star formation. These results are supported by model comparison with very faint colour distributions. We find good agreement with the no-evolution model in the range b(_j) – R(_j) > 1.6, indicating a minimum of evolution of bright early type galaxies. The excess number count is clearly concentrated blueward of this, and consistent with the expectation of the burst model. Clustering on small scales, estimated via the 2-point spatial correlation function agrees fairly well with local results, but evidence for a very large scale periodicity may be indicated by combining this survey with others in the direction of the Galactic poles.