Coma Cluster Treasury Survey

Project Overview - Objective 4

Color gradients and Stellar Populations

The steepness of the radial color gradients in dwarf and giant galaxies directly reflects their merging history: the "monolithic collapse" model imposes an initially steep metallicity gradient, which will be progressively diluted by subsequent galaxy merger events. Semi-analytic models of such hierarchical galaxy formation predict differences in the merger history as a function of galaxy type, mass, and environment. In order to provide a complete understanding of dwarf galaxies in clusters, it is crucial to acquire a sizable and unbiased sample, so that the global "scaling laws" of different dwarf galaxy subtypes (dE, dS0, dE-N, dIrr) can be reliably determined. Giant elliptical galaxies in the Coma cluster have metallicity gradients that correlate with their mass, which is broadly consistent with the monolithic collapse models. In contrast, the dwarf elliptical galaxies display a variety of colour gradients. The Coma Cluster Treasury Survey will provide the first dataset to test detailed predictions for "scaling laws" and internal colour gradients in a rich cluster environment, for a large range of galaxy masses. Complemented by near-infrared images and two-dimensional spectral information from other telescopes, our HST images will be used to assess whether the observed colour gradients are due to changes in age and/or metallicity from the centre to the edge of the galaxies.

Another major outstanding puzzle is the physical origin of post-starburst spectra in distant clusters. Spectroscopic surveys have identified several galaxies with post-starburst spectra in the infall region of the Coma cluster. Suggestions as to what has triggered these bursts range from equal-mass galaxy mergers to high-speed interaction with the dense intracluster medium (i.e. gas). The key to distinguishing between these processes is the spatial distribution of the intermediate-age populations. It would be concentrated toward the center if major mergers were involved, while it would be an extended phenomenon if the galaxy had swept through the intracluster medium. The sharp eye of HST is needed to provide a clear-cut answer on this issue, which could provide a breakthrough in our understanding of the evolutionary effects observed in distant clusters.