Formation and evolution of red Hδ-strong galaxies in distant clusters:: Two different evolutionary paths to S0 galaxies?

We numerically investigate the spectrophotometric evolution of galaxies with a variety of different star formation histories and degrees of dust extinction in order to elucidate the origin of the red Hdelta-strong'' (RHDS) galaxies-characterized by strong [EW(Hdelta)>3 Angstrom] Balmer line absorption but with colors as red as the dormant E/S0 systems-first observed in distant clusters of galaxies. Specifically, we investigate the evolution in the [(B-R), EW(Hdelta)]-plane of two different galaxy models: a truncation'' model in which a galaxy with a declining star formation rate (typical of a normal spiral) has its star formation abruptly truncated, and a starburst'' model in which a galaxy undergoes a secondary burst of star formation that is abruptly halted. This leads to the following three main results: (1) Both models pass through the zone inhabited by the RHDS galaxies in this plane, but the truncation'' models cannot explain the most extreme cases, which have EW(Hdelta) >6 Angstrom. (2) The reddest RHDS galaxies can only be explained by truncation'' or starburst'' models that have very heavy dust extinction (A(v)>0.5 mag). (3) Irrespective of whether dust effects are included, the truncation'' models are incapable of reproducing the very blue (B-R<1.6) analogs of the RHDS galaxies. By comparing these results with recent Hubble Space Telescope-based morphological and ground-based spectroscopic studies of distant cluster galaxies, we propose that there are two possible different evolutionary paths for the transformation of cluster spirals into S0 galaxies.