STAR-FORMATION IN EARLY-TYPE GALAXIES IN THE COMA CLUSTER

We have obtained multifiber spectra of 125 early-type galaxies in the Coma cluster of galaxies, using the Hydra bench spectrograph on the 4 m telescope at Kitt Peak. The sample selection was based on both color and morphology, care being taken to exclude intermediate and late-type spirals. Surprisingly, a large fraction (about 1/3) of all galaxies in a field located approximately 40' SW of the center of the cluster exhibits enhanced Balmer absorption lines or emission lines, indicative of recent star formation or nuclear activity. In contrast, very few such galaxies are found in the central field of the cluster. Those galaxies with abnormal spectra cover a range in luminosity down to B almost-equal-to 19 (M(B) almost-equal-to -15). Furthermore, the bulk of the abnormal-spectrum galaxies are located near a secondary peak in the cluster x-ray emission found by Briel et al. [A&A, 259, L31 (1992)] and by Watt et al. [MNRAS, 258, 738 (1992)]. We argue by detailed comparisons with Kennicutt's [ApJS, 79, 255 (1992a)] sequence of normal spiral galaxy spectra that the anomalous spectral features cannot arise from morphological misindentifications. Specifically, the abnormal spectra are fundamentally different, with [O II] emission lines too weak for the enhanced Balmer absorption. The spectra are, however, remarkably similar to the red Hdelta-strong galaxies studied by Couch & Sharples [MNRAS, 229, 423 (1987)] in clusters at redshift z almost-equal-to 0.3 Hence it appears that an environmental process similar to that frequently found in clusters at moderate redshift is occurring at the present epoch in the outskirts of the Coma cluster.