THE DWARF SPHEROIDAL COMPANIONS TO M31 - SURFACE-BRIGHTNESS PROFILES

We present CCD images and surface photometry for the three dwarf spheroidal companions to M31. The derived light profiles are the most accurate to date for galaxies of this type and serve to define the faint end of the surface-brightness-luminosity relation for early type dwarfs. We also compute new structural parameters for a subset of the Galactic dwarf spheroidals, which indicate, when compared to the M31 dwarfs, that both sets of dwarf galaxies are similar. A B - V color is measured for And I and found to be in accord with that expected from the metallicity derived by Mould and Kristian from their color-magnitude diagram study. However, to a high degree of accuracy, we rule out their claim of a color gradient in this galaxy. The metallicity-luminosity relation for E galaxies is investigated using these and other new data. The results suggest that a change in slope occurs between the dwarfs (Z approximately L0.4) and the more luminous E galaxies (Z approximately L0.6). Furthermore, our results, together with other available data, confirm earlier suggestions that the correlation between central surface brightness and luminosity for dE galaxies breaks down for central surface brightnesses fainter than approximately 24th V mag per square arcsec; below this limit dE galaxies are found over a wide range of luminosities.