DEEP CCD PHOTOMETRY OF THE RICH GALAXY CLUSTER ABELL 1656 CHARACTERISTICS OF THE DWARF ELLIPTICAL GALAXY POPULATION IN THE CLUSTER CORE

Abstract

<p>We have developed a statistically rigorous and automated method to implement the detection, photometry and classification of faint objects on digital images. We use these methods to analyze deep R- and B-band CCD images of the central ~ 700 arcmin² of the Coma cluster core, and an associated control field. We have detected and measured total R magnitudes and (B - R) colors for a sample of 3741 objects on the galaxy cluster fields, and 1164 objects on a remote control field, complete to a limiting magnitude of R = 22.5 mag. The typical uncertainties are ±0.06 and ±0.12 mag in total magnitude and color respectively. The dwarf elliptical (dE) galaxies are confined to a well-defined sequence in the color range given by 0.7 ≤ (B - R) ≤ 1.9 mag; within this interval there are 2535 dE candidates on our fields in the cluster core, and 694 objects on the control field. With an image scale of 0.53 arcsec/pixel and seeing near 1.2 arcsec, a large fraction of the dE galaxy candidates are resolved.</p> <p>We find a significant metallicity gradient in the radial distribution of the dwarf elliptical galaxies, which goes as Z ∝ R⁻⁰˙³² outwards from the cluster center at NGC 4874. As well, there is a strong color-luminosity correlation, in the sense that more luminous dE galaxies are redder in the mean. These effects give rise to a radial variation in the cluster luminosity function. The spatial distribution of the faint dE galaxies is well fit by a standard King model with a central surface density of ∑₀=1.44 dEs arcmin⁻², a core radius Rc = 18.7 arcmin (≃ 0.44 Mpc), and a tidal radius of 1.44 deg (≃ 2.05 Mpc). This core is significantly larger than Rc = 12.3 arcmin (≃ 0.29 Mpc) found for the bright cluster galaxies. The composite luminosity function for Coma galaxies is modeled as the sum of a log-normal distribution for the giant galaxies and a Schechter function for the dwarf elliptical galaxies, with a faint-end slope of α = -1.41, consistent with known faint-end slopes for tbe Virgo and Fornax clusters. The early-type dwarf-to-giant ratio for the Coma cluster core is consistent with that of the Virgo cluster, and thus with the rich Coma cluster being formed as the merger of multiple less-rich galaxy clusters.</p>