7 Conclusions

A new code Z-PEG is proposed for public use, available on the web, to predict photometric redshifts from a data set of classical broad-band colors or spectra. The code is derived from the model PÉGASE, also available on the same web site, frequently downloaded for evolution spectrophotometric analyses and quoted in many articles. The particularity of the analysis presented here is to underline the hypotheses implicitly (or explicitly) accepted when computing photometric redshifts with evolution. The comparison with a spectrophotometric redshift sample makes the predictions robust. In its recent version PÉGASE.2, the model constrains typical evolution scenarios to fit nearby galaxies. Type-dependent extinction, IMF and metal effects are taken into account, although we assumed their modeling is not essential in our conclusions. The most constraining parameter is the z=0 galaxy age, typical of the nearby stellar population of each spectral type. This constraint makes it possible to eliminate implausible secondary solutions, the degeneracy being the largest cause of uncertainties on photometric redshifts. The dispersion for z <1.5reaches its minimum value 0.0980 with the age constraint, compared to other dispersion values whatever the other parameter values (IGM absorption or NIR colors). The improvement in photometric redshift accuracy brought by near-infrared colors is also measured by comparing models Z-PEG.4 and Z-PEG.2. The dispersion decreases from 0.32 to 0.12 for z < 1.5 and is more important for high redshifts when the 4000 Å discontinuity enters in the NIR domain and the Lyman break does not yet reach the ultraviolet bands. Finally Z-PEG is proposed to the community with PÉGASE.2. The site will be updated and improved, so that Z-PEG will also follow improvements in the spectrophotometric evolution modeling, in particular with the series of flux-calibrated samples allowing better fits of highly redshifted templates. A possible extension of Z-PEG will involve the adaptation of any photometric system, including narrow bands around emission lines, with the help of the coupled code PÉGASE+CLOUDY (Ferland 1996), predicting stellar and photoionised gas emissions (Moy et al. 2001).

Acknowledgements

We would like to thank Emmanuel Moy, Michel Fioc and Jérémy Blaizot for the fruitful discussions we had with them. We are also grateful to the referee for his/her useful comments.