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 F300WAB, F450WAB, F606WAB and F814WAB number counts for galaxies in our catalogue (Volonteri et al. 2000): the number-counts relation has an increasing slope up to the limits of the survey in all four bands. The slope is steeper at shortest wavelengths: we estimated γF300WAB ∼ 0.47± 0.05, γF450WAB ∼ 0.35 ± 0.02, γF606WAB ∼ 0.28 ± 0.01 and γF814WAB ∼ 0.28 ± 0.01. The color-magnitude relations of galaxies shows an initial blueing trend, 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 sources with F814WAB > 27 having (F450W-F606W)AB bluer than a typical local irregular galaxy. Morphological number counts are actually dominated by late type galaxies, while early type galaxies show a decreasing slope at faint magnitudes. Combining this information with photometric redshifts, we notice that galaxies contributing with a steep slope to the number counts have z & 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 galaxies along the Hubble sequence.