FUNDAMENTAL PARAMETERS OF BRIGHTEST CLUSTER GALAXIES

We have obtained photometry and spectroscopy of a sample of brightest cluster galaxies in order to determine the relationships between scale length, surface brightness, velocity dispersion, and line strength, and to make comparisons with the relations determined for lower luminosity elliptical galaxies. We have computed the velocity dispersions for 32 brightest cluster galaxies, which when supplemented by additional data from the literature, give us a sample of 43 galaxies. CCD surface photometry of these galaxies produced characteristic scale lengths and surface brightnesses. We find that the relation between log r(e), log sigma, and mean surface brightness reported by Dressler et al. for lower luminosity ellipticals also describes the relation for brightest cluster galaxies fairly well. There is a slight curvature in the fundamental plane at the high-luminosity end, apparently due to the fact that the velocity dispersions of BCGs increase more slowly with luminosity than those for lower luminosity ellipticals. We derive the fundamental plane solutions for BCGs in r(e) - sigma - <mu>e space, which can be used as a distance indicator with a predicted error of approximately 21% per galaxy. The r(e) - <mu>e relation for BCGs is also a good distance indicator (distance errors of approximately 25% per galaxy) and has the advantage of not requiring knowledge of the velocity dispersion. We find only a weak correlation between the Na D 5893 line strength (metallicity) and velocity dispersion for BCGs, in contrast to the significant correlation between these quantities reported for lower luminosity ellipticals. Finally, we find that the mean mass-to-light ratio for BCGs is a factor of approximately 2 greater than for the lower luminosity ellipticals in the Virgo and Coma sample of Dressler et al., consistent with the correlation M/L approximately L1/4. By observing BCGs at higher redshift (i.e., z approximately 0.5), the fundamental plane solutions can be used to determine the change in surface brightness due to stellar evolution with an accuracy of approximately 25% and to place limits on q0.