Formation and Evolution of Red Hδ‐strong Galaxies in Distant Clusters: Two Different Evolutionary Paths to S0 Galaxies?

Abstract

We numerically investigate the spectrophotometric evolution of galaxieswith a variety of different star formation histories and degreesof dust extinction in order to elucidate the origin of the ``redHδ-strong'' (RHDS) galaxies-characterized by strong [EW(Hδ)>3 Å]Balmer line absorption but with colors as red as the dormant E/S0systems-first observed in distant clusters of galaxies. Specifically,we investigate the evolution in the [(B-R), EW(Hδ)]-plane of twodifferent galaxy models: a ``truncation'' model in which a galaxywith a declining star formation rate (typical of a normal spiral)has its star formation abruptly truncated, and a ``starburst'' modelin which a galaxy undergoes a secondary burst of star formation thatis abruptly halted. This leads to the following three main results:(1) Both models pass through the zone inhabited by the RHDS galaxiesin this plane, but the ``truncation'' models cannot explain the mostextreme cases, which have EW(Hδ)>6 Å. (2) The reddest RHDS galaxiescan only be explained by ``truncation'' or ``starburst'' models thathave very heavy dust extinction (Av>0.5 mag). (3) Irrespective ofwhether dust effects are included, the ``truncation'' models areincapable of reproducing the very blue (B-R<1.6) analogs of the RHDSgalaxies. By comparing these results with recent Hubble Space Telescope-basedmorphological and ground-based spectroscopic studies of distant clustergalaxies, we propose that there are two possible different evolutionarypaths for the transformation of cluster spirals into S0 galaxies.