Starbursts versus Truncated Star Formation in Nearby Clusters of Galaxies

We present long-slit spectroscopy, B and R bandpass imaging, and 21 cmobservations of a sample of early-type galaxies in nearby clusters which areknown to be either in a star-forming phase or to have had star formation whichrecently terminated. From the long-slit spectra, obtained with the Blanco 4-mtelescope, we find that emission lines in the star-forming cluster galaxies aresignificantly more centrally concentrated than in a sample of field galaxies.The broadband imaging reveals that two currently star-forming early-typegalaxies in the Pegasus I cluster have blue nuclei, again indicating thatrecent star formation has been concentrated. In contrast, the two galaxies forwhich star formation has already ended show no central color gradient. ThePegasus I galaxy with the most evident signs of ongoing star formation(NGC7648), exhibits signatures of a tidal encounter. Neutral hydrogenobservations of that galaxy with the Arecibo radiotelescope reveal the presenceof ~4 x 10^8 solar masses of HI. Arecibo observations of other current orrecent star-forming early-type galaxies in Pegasus I indicate smaller amountsof gas in one of them, and only upper limits in others.Comment: to be published in Astronomical Journa