Photometry and Spectroscopy of Coma Dwarf Ellipticals

Abstract

We have used deep R- and B-band CCD images (with ~1.2 arcsec seeing) of the central ~700 arcmin^2 of the Coma cluster core (and an associated ~270 arcmin^2 control field) to study the early-type dwarf elliptical (dE) galaxy population. Analysis of the color-restricted control-field-corrected color-magnitude diagram (CMD) shows that the dE galaxy sequence is linear over the range 15.5 <= R <= 22.5 mag, with a slope Delta (B-R)/Delta R = -0.056+/-0.002. The faintest dEs in this sample have a color (B-R) = 1.15 mag, which is within the color range measured for the diffuse intracluster light. Analysis of the completeness-corrected background-subtracted radial number density profiles reveals that the faint dEs are deficient in the cluster core, consistent with their destruction in this dense environment. To determine ages and abundances for a subsample of these Coma dEs, we used the Keck I telescope and the Low-Resolution Imaging Spectrograph (LRIS) in ~1.0 arcsec seeing. We obtained 2x1800 sec exposures of four fields, with ~22 objects per field. Our wavelength coverage is 3800 < λ< 6200 Å, with 8 Åresolution and S/N~50 for the dEs. Preliminary analysis of an LRIS field located ~3.8 arcmin west of NGC 4874 reveals that only four of the 17 target objects have Coma velocities, compared to 9-10 expected from analysis of the control-field CMD.