A New Model of Spectral Evolution: PEGASE Application to Galaxy Counts

A new model of spectral evolution of galaxies (Fioc & Rocca-Volmerange 1996) named PEGASE (in french, Projet d’Etude des GAlaxies par Synthese Evolutive) has been developped. It uses the principle of isochrone synthesis and allows to compute synthetic spectra of galaxies on timescales as short as 1 Myr for starbursts or as long as 20 Gyrs for Hubble sequence galaxies. Its wavelength range extends from the far UV (200 A) to the NIR (3 μm). A new library of stellar spectra, mainly observational, has been built. Isochrones are computed for 2 sets of tracks with solar metallicity (Padova and Genova) up to the EAGB completed by the TPAGB phase (Groenewegen et al., 1993) and the PAGB phase from Schonberner (1983) and Blocker (1995). Nebular emission (continuum and lines), extinction in disks and in spheroids computed from the metallicity of the ISM and ejecta from stars are also considered.