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The Impact of Starbursts and Post-Starbursts on the Photometric Evolution of High Redshift Galaxies

We present evolutionary synthesis models for galaxies of spectral types Sa through Sd with starbursts of various strengths triggered at various redshifts and study their photometric evolution before, during, and after their bursts in a cosmological context. We find that bursts at high redshift, even very strong ones, only cause a small blueing of their intrinsically blue young parent galaxies. At lower redshift, even small bursts cause a significant blueing of their intrinsically redder galaxies. While the burst phase is generally short, typically a few hundred Myr in normal-mass galaxies, the postburst stage with its red colors and, in particular the very red ones for early bursts at high redshift, is much longer, of the order of several Gyr. Even without any dust, which in the postburst stage is not expected to play an important role anyway, models easily reach the colors of EROs in the redshift range z=2 through z=0.5 after starbursts at redshifts between 2 and 4. We therefore propose a third alternative for the ERO galaxies beyond the two established ones of passive galaxies vs. dusty starbursts: the dust-free post-(strong-)starbursts. A very first comparison of our models to HDF data with photometric redshifts shows that almost all of the outliers that could not be described with our chemically consistent models for undisturbed normal galaxy types E through Sd can now be explained very well. Galaxies in the redshift range from z=2.5 to z=0.5 that are redder, and in some cases much redder, than our reddest undisturbed model for a high-metallicity classical elliptical are well described by post-starburst models after starbursts at redshifts between 2 and 4. Galaxies bluer than our bluest low metallicity Sd model, most of which have redshifts lower than 1, are well explained by ongoing starbursts.