The Detailed Evolution of E+A Galaxies into Early Types

Post-starburst, or E+A galaxies, are the best candidates for galaxies in transition from being gas-rich and star-forming to gas-poor and passively-evolving via galaxy-galaxy mergers. To determine what E+A galaxies become after their young stellar populations fade away, we present the detailed morphologies of 21 E+As using HST images. We find that E+As are similar to early types in that they have large bulge fractions (median B/T = 0.59), high Sersic index (n > 4), and high concentration indices (C > 4.3). The large fraction (70%) of E+As with positive color gradients (i.e., bluer nuclei) indicates that the young stellar populations are more concentrated than the old populations. We show that these positive color gradients can evolve into the negative gradients typical in E/S0s if the central parts of these galaxies are metal enhanced. E+A galaxies stand apart from the E/S0s in the edge-on projection of the Fundamental Plane, implying that E+As have, on average, a M/L that is ~3.8 times smaller than that of E/S0s. The tilt of the E+A FP indicates that the variation among stellar populations in these galaxies is closely tied to their structural parameters such that smaller or less massive galaxies have smaller M/L. We find a population of unresolved compact sources in nine E+As (45%). Their colors and luminosities are consistent with the hypothesis that these are newly formed star clusters. The bright end of the cluster LF is fainter in redder E+A's, suggesting that the young star cluster systems have faded or been disrupted as the merger remnant aged. In summary, the morphologies, color profiles, scaling relations, and cluster populations are all consistent with the hypothesis that E+As galaxies are the results of mergers that evolve into early-type galaxies.