Interpreting the optical data of the Hubble Deep Field South: colors, morphological number counts and photometric redshifts

We present an analysis of the optical data of the Hubble Deep Field South. We derive F300W(AB), F450W(AB), F606W(AB) and F814W(AB) number counts for g alaxies in our catalogue (Volonteri et al. 2000): the number counts relatio n has an increasing slope up to the limits of the survey in all four bands. The slope is steeper at shortest wavelengths: we estimated gamma (F300WAB) similar to 0.47 +/- 0.05, gamma (F450WAB) similar to 0.35 +/- 0.02, gamma (F606WAB) similar to 0.28 +/- 0.01 and gamma (F814WAB) similar to 0.28 +/- 0.01. The color-magnitude relations of galaxies shows an initial blueing tr end, which gets flat in the faintest magnitude bin and the sample contains a high fraction of galaxies bluer than local sources, with about 50% of sou rces with F814W(AB) > 27 having (F450W-F606W)(AB) bluer than a typical loca l irregular galaxy. Morphological number counts are actually dominated by l ate type galaxies, while early type galaxies show a decreasing slope at fai nt magnitude. Combining this information with photometric redshifts, we not ice that galaxies contributing with a steep slope to the number counts have z greater than or similar to 1, suggesting a moderate merging. However we emphasize that any cut in apparent magnitude at optical wavelengths results in samples biased against elliptical galaxies, affecting as a consequence the redshift distributions and the implications on the evolution of galaxie s along the Hubble sequence.