Starbursts versus truncated star formation in nearby clusters of galaxies

We present long-slit spectroscopy, B- and R-bandpass imaging, and 21 cm obs ervations of a sample of early-type galaxies in nearby clusters, which are known to be either in a star-forming phase or to have had star formation th at recently terminated. From the long-slit spectra, obtained with the Blanc o 4 m telescope, we find that emission lines in the star-forming cluster ga laxies are significantly more centrally concentrated than in a sample of fi eld galaxies. The broadband imaging reveals that two currently star-forming early-type galaxies in the Pegasus I cluster have blue nuclei, again indic ating that recent star formation has been concentrated. In contrast, the tw o galaxies for which star formation has already ended show no central color gradient. The Pegasus I galaxy with the most evident signs of ongoing star formation (NGC 7648), exhibits signatures of a tidal encounter. Neutral hy drogen observations of that galaxy with the Arecibo radio telescope reveal the presence of similar to4 x 10(8) M. of H I. Arecibo observations of othe r current or recent star-forming early-type galaxies in Pegasus I indicate smaller amounts of gas in one of them, and only upper limits in others. The se observations indicate that NGC 7648 in the Pegasus I cluster owes its pr esent star formation episode to some form of tidal interaction. The same ma y be true for the other galaxies with centralized star formation, but we ca nnot rule out the possibility that their outer disks have been removed via ram pressure stripping, followed by rapid quenching of star formation in th e central region.