Photometric redshifts from evolutionary synthesis with PEGASE:: The code Z-PEG and the z=0 age constraint

Photometric redshifts are estimated on the basis of template scenarios with the help of the code Z-PEG, an extension of the galaxy evolution model PEGASE.2 and available on the PEGASE web site. The spectral energy distribution (SED) templates are computed for nine spectral types including starburst, irregular, spiral and elliptical. Dust, extinction and metal effects are coherently taken into account, depending on evolution scenarios. The sensitivity of results to adding near-infrared colors and IGM absorption is analyzed. A comparison with results of other models without evolution measures the evolution factor which systematically increases the estimated photometric redshift values by Deltaz greater than or equal to 0.2 for z >1.5. Moreover we systematically check that the evolution scenarios match observational standard templates of nearby galaxies, implying an age constraint of the stellar population at z = 0 for each type. The respect of this constraint makes it possible to significantly improve the accuracy of photometric redshifts by decreasing the well-known degeneracy problem. The method is applied to the HDF-N sample (see in Fernandez-Soto et al. 1999). From fits on SED templates by a chi(2)-minimization procedure, not only is the photometric redshift derived but also the corresponding spectral type and the formation redshift z(for) when stars first formed. Early epochs of galaxy formation z(for) > 5 are found from this new method and results are compared to faint galaxy count interpretations. The new tool is available at: http://www.iap.fr/pegase.