Date of Award
Doctor of Philosophy (PhD)
William E. Harris
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.
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.
Secker, Jeff, "DEEP CCD PHOTOMETRY OF THE RICH GALAXY CLUSTER ABELL 1656 CHARACTERISTICS OF THE DWARF ELLIPTICAL GALAXY POPULATION IN THE CLUSTER CORE" (1995). Open Access Dissertations and Theses. Paper 3997.