6 Discussion

The determination of photometric redshifts significantly depends on the adopted evolution scenarios. The basic principle is that distant galaxies evolve according to the same basic principles as our nearby galaxies, only observed at earlier epochs. As a direct consequence, evolution scenarios must reproduce the SEDs of galaxy templates observed at z=0. This constraint requires meaningful statistical templates. The literature appears poor in this domain. From a large series of catalogs, we built statistical samples corrected for various effects: aperture, inclination, reddening, etc. (see Fioc & Rocca-Volmerange 1999b) in order to focus specifically on visible to near-infrared colors of eight spectral types of galaxies. All our redshifted templates are compatible with these statistical colors at z=0. The evolution principles are linked to the time-dependent star formation rates. Corresponding to so-called "monolithic'' scenarios, each type evolves with a star formation rate proportional to the gas content and typical astration rates. The time-scale of star formation increases from ellipticals (half Gyr) to Irregulars (more than 10 Gyr). A test of the reliability of the adopted scenarios is given with the interpretation of the deepest multi-spectral faint galaxy counts on the largest multi-spectral coverage. Figure 5 of Fioc & Rocca-Volmerange (1999a) shows the best simultaneous fits of the deepest surveys, including the HDF-N sample, from the far-UV to the K band. The most constraining data are the reddest and deepest counts, only fitted by scenarios of old elliptical galaxy models. The evolution scenarios are from PÉGASE and similar to those used in the code Z-PEG. These considerations make the $z_{{\rm Z-PEG}}$ results more robust and the evolution scenarios of elliptical galaxies compatible with a rapid evolution at the earliest epochs. The high values of $z_{{\rm for}}$ found from the present analysis favor this result. The star formation rate continuously follows the gas density, so that the current SFR depends on the past star formation history. An interesting interpretation, though tentative given the small incomplete samples, would arise if the apparent continuous law is considered as the envelop of small bursts. The intensity of these bursts decreases over time, as the gas is depleted by star formation.

If the photometric redshifts $z_{{\rm Z-PEG}}$ are faintly dependent on the adopted cosmology, the results of $z_{{\rm for}}$ are more sensitive to the choice of the cosmological parameters through the age-redshift relation.

Finally the discussion of the dispersion and the degeneracy of solutions has been detailed at each step of the $z_{\rm phot}$ determination. In this analysis, like the age constraint, the extension of the wavelength coverage, increasing with the number of filters, limits the degeneracy. Conversely, if galaxies are observed through only a few filters, the degeneracy may be high and results have to be used with caution.