Dissecting the Luminosity Function of the Coma Cluster of Galaxies Using Canada‐France‐Hawaii Telescope1 Wide‐Field Images

Abstract

We determined the relative spatial density of the Coma Cluster galaxies, selected by luminosity and by central brightness, i.e., the luminosity function bivariate in central brightness. The Coma Cluster and control fields were imaged using the CFH12K (42' × 28') and UH8K (28' × 28') wide-field cameras at the Canada-France-Hawaii Telescope. Selected Hubble Space Telescope (HST) images were used for testing. Quantities were derived from measurements in at least two colors, which have the following features: (1) galaxies as faint as 3 times the luminosity of the brightest globular clusters are in the completeness region of our data. (2) We have a complete census (in the explored region) of low surface brightness galaxies with a central surface brightness almost as low as the faintest ones so far cataloged. (3) The explored area is among the largest ever sampled with CCDs at comparable depth for any cluster of galaxies. (4) The error budget includes all sources of errors known to date. Using HST images, we also discovered that blends of globular clusters, not resolved into individual components due to seeing, look like dwarf galaxies when observed from the ground and are numerous and bright. When mistaken as extended sources, they increase the steepness of the luminosity function at faint magnitudes. The derived Coma luminosity function is relatively steep (α = -1.4) over the 11 magnitudes sampled, but the slope and shape depend on color. A large population of faint low surface brightness galaxies was discovered, representing the largest contributor (in number) to the luminosity function at faint magnitudes. We found a clear progression for a faintening of the luminosity function from high surface brightness galaxies (μ ~ 20 mag arcsec-2) to galaxies of very faint central brightness (μ ~ 24.5 mag arcsec-2), and some evidence for a steepening. Compact galaxies, usually classified as stars and therefore not included in the luminosity function, are found to be a minor population in Coma.