M87, Globular Clusters, and Galactic Winds: Issues in Giant Galaxy Formation

Abstract

We have used the high-resolution camera at the Canada-France-Hawaii Telescope to obtain VRI photometry of the globular clusters in the innermost 140'' of the M87 halo. The results are used to discuss several issues concerning the formation and evolution of globular cluster systems in supergiant elliptical galaxies like M87. Our principal results are as follows: (1) From our deep R-band photometry of the cluster population, we find no significant change in the globular cluster luminosity function (GCLF) with galactocentric radius, for cluster masses M 105 M☉. This result places constraints on current theoretical predictions of the rate of cluster evolution by tidal shocking and evaporation, indicating that the main effects of dynamical evolution may be felt only on lower mass clusters (105 M☉) that are below the faint limit of most current observations. (2) Combining our V - I color indices with other data in the literature, we derive the metallicity gradient and mean metallicity of the clusters from r = 9'' out to r 500'' (~0.7–35 kpc). Within the core radius rc = 1' of the globular cluster system, the metallicity distribution is uniform, but at larger radii the mean metallicity declines steadily as Z/Z☉ ~ r-0.9. (3) The various options for explaining the existence of high specific frequency galaxies like M87 are evaluated. We argue that alternatives involving secondary evolution (such as the formation of many globular clusters during mergers, or the existence of a population of intergalactic globular clusters centered on the same location as the central elliptical galaxy) are capable of modifying the characteristics of the globular cluster system in distinctive ways, but are not likely to be the primary explanation for high-SN galaxies. (4) We offer a new explanation for the large observed SN range among the brightest cluster elliptical galaxies (BCGs). It is suggested that these central supergiant E galaxies formed in an exceptionally turbulent or high-density environment which favored a very rapid initial star formation burst. As a result, a much higher than average fraction of the protogalactic gas was driven out in a galactic wind not long after the first globular clusters were formed, thus artificially boosting the specific frequency of the remaining galaxy. From a total sample of 30 BCGs, we derive empirical scaling relations that relate to this hypothesis. Our analysis favors the view that BCGs began forming at redshifts z 5, distinctly earlier than most other galaxies.