The Early-type Dwarf-to-Giant Ratio and Substructure in the Coma Cluster

Abstract

We have obtained new CCD photometry for a sample of $\simeq 800$ early-type galaxies (dwarf and giant ellipticals) in the central 700 arcmin${}^2$ of the Coma cluster, complete in color and in magnitude to $R=22.5$ mag ($M_R\simeq -12$ mag for $H_0=86$ km/sec/Mpc). The composite luminosity function for all galaxies in the cluster core (excluding NGC 4874 and NGC 4889) is modeled as the sum of a Gaussian distribution for the giant galaxies and a Schechter function for the dwarf elliptical galaxies. We determine that the early-type dwarf-to-giant ratio (EDGR) for Coma is identical to that measured for the less rich Virgo cluster; i.e., the EDGR does not increase as predicted by the EDGR-richness correlation. We postulate that the presence of substructure is an important factor in determining the cluster's EDGR; that is, the EDGR for Coma is consistent with the Coma cluster being built up from the merger of multiple less-rich galaxy clusters.

13 pages, LaTeX file, 3 EPS figures, uses aaspp4.sty; also available from: http://www.sci.wsu.edu/math/faculty/secker/secker.html; to be published in ApJ, 469 (Oct.1, 1996)

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