Spectra of galaxies in clusters. II - Models for the spectra of postburst and stripped galaxies

We have computed models for the spectral evolution of early-type galaxies undergoing starbursts and for a model of a late-type galaxy where stripping halts star formation. The goal of this work is to investigate the peculiar combination of mean Balmer absorption strengths, (H) = 7-8 A, and relatively red color, B-V = 0.74 +/- 0.1, observed in the "E + A" spectra of galaxies in 3C 295 and other high-redshift rich clusters. We find that bursts consuming ~ 3%-8% of the luminous mass of the underlying galaxy match the observed and B-V quite well. These values are also produced by a stripped model which terminates star formation over a time of ~0.1 Gyr or less. Since both types of models match the data, there is no strong reason to reject the stripping picture in favor of starbursts occurring in the densest clusters. We have computed a number of line and band absorption and continuum indices which can be measured in the optical spectra of faint high-redshift galaxies. Bursts having a constant star formation rate of ~ 0.5 Gyr duration cannot be distinguished from the stripped model in any of the indices that were compared. However, bursts that do not form O-B2 stars and have "delayed" star formation can be distinguished from the stripped model in comparisons between and the 4000 A break, H+K, and G band indices. Since the 4000 A break is affected by the internal reddening in a galaxy, the H+K and G band indices are the best discriminators of star formation history. The separation between the various models is not large in any indices and a measurement precision of at least 5% is required to discriminate between them. We suggest that many of the E+A galaxies observed in high-redshift clusters might be recently stripped, while those in the high-redshift field are experiencing starbursts.