The structure of elliptical and cD galaxies.

A method of photographic photometry is described which permits surface photometry of galaxies to be extended to very low (5v > 28 mag arcsec -2) surface brightnesses. It has been used to study ellfptical galaxies in three clusters and a number of cD galaxies, with the following results. Ignoring flattening, elliptical galaxies form a one-parameter family with light distributions which are well described by an exponentially truncated Hubble law. Their limiting radii R1 L0213, where L0 is an envelope-independent luminosity. There is no evidence that R1 is determined by tidal interactions with other galaxies. The core radii of ellipticals decrease with decreasing luminosity, but make a sudden jump at Mv -15. The cD galaxies and other elliptical brightest cluster members appear to be very bright-but otherwise normal-giant elliptical galaxies with very diffuse, extended envelopes superposed. The extent and luminosity of these outer envelopes is a strong function of cluster richness; in the most extreme case the envelope extends to a radius of 2 Mpc. There appears to be some hope of explaining the occurrences and properties of cD galaxies by the effects of dynamical friction within clusters. Subject headings: galaxies: formation - galaxies: photometry - galaxies: structure