Project Overview - Objective 7

The Nature of E+A Galaxies

Spectroscopic surveys have identified several galaxies with post-starburst spectra in both the core and the infall region of the Coma cluster. These galaxies are referred to as "E+A" galaxies, with the "E" representing an Elliptical galaxy (which have old stars, roughly 10 billion years old) and the "A" denoting the presence of intermediate-age A-type stars. "E+A" galaxies have experienced a burst of star formation which was turned off within the last billion years.

Suggestions as to what may have triggered these bursts of star formation range from equal-mass galaxy mergers to shock-generating, high-speed impacts of infalling groups of galaxies into the clusters's halo of hot gas. The key to distinguishing between these processes resides with the spatial distribution, within the cluster, of systems with these intermediate-age stellar populations. They would be concentrated to the center of the galaxy cluster if major mergers were involved, while it would be either an extended or off-center phenomenon if the galaxies had experienced an interaction with the hot intracluster gas. The Coma Cluster Treasury Survey includes a number of "E+A" galaxies at a range of luminosities, and the morphological features and color maps generated from the two HST filters should also enable a valuable diagnostic check on the physical origin of the "E+A" phenomenon.