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

Abstract

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 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 gamma _{F300W_AB} ~ 0.47+/-0.05, gamma _{F450W_AB} ~ 0.35+/-0.02, gamma _{F606W_AB} ~ 0.28+/-0.01 and gamma _{F814W_AB} ~ 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 F814W_AB>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 zgtrsim1 , 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.