Galaxy counts, sizes, colours and redshifts in the Hubble Deep Field

We compare the galaxy evolution models of Bruzual & Charlot (1993) with faint galaxy count, size and colour data from the Hubble and Herschel Deep Fields (Metcalfe et al 1996). For q(0) = 0.05, we find that models where the SFR increases expodentially out to z > 2 are consistent with all of the observational data. For q(0) = 0.5, such models require an extra population of galaxies which are only seen at high redshift and then rapidly fade or disappear. We find that, whatever the cosmology, the redshift of the faint blue galaxies and hence the epoch of galaxy formation is likely to lie at z greater than or similar to 2. We find no implied peak in the SFR at z approximate to 1 and we suggest that the reasons for this contradiction with the results of Madau et al (1996) include differences in faint galaxy photometry, in the treatment of spiral dust and in the local galaxy count normalisation.