Galaxy number counts and cosmology

Using COSMOS machine measurements of UK Schmidt and Anglo-Australian telescope photographs in a blue and a red passband we have obtained galaxy number counts and colour distributions to faint (B ∼ 23 mag) limits. We compare our counts with those of other authors and find good agreement with the counts of Kron, Koo and Couch & Newell. We have modelled our observations and find strong evidence for galaxy luminosity evolution in the blue counts and in the faint galaxy colour distribution. The red counts show evidence for slower rates of galaxy luminosity evolution and, here, the effects of evolution and q0 are comparable. The red count models are well enough determined to reject world models with q0 ≳ 0.5, if galaxies were known to have undergone little luminosity evolution over a look-back time corresponding to a redshift of 0.5. We discuss the possibility of using a well-determined Hubble diagram to further separate out the effects of q0 and luminosity evolution in the red number counts. Similar constraints may be obtained from faint R ∼ 21 mag galaxy redshift surveys when these become observationally feasible.