The Detailed Evolution of E+A Galaxies into Early Types

Post-starburst, or E+A galaxies, are the best candidates for galaxies intransition from being gas-rich and star-forming to gas-poor andpassively-evolving via galaxy-galaxy mergers. To determine what E+A galaxiesbecome after their young stellar populations fade away, we present the detailedmorphologies of 21 E+As using HST images. We find that E+As are similar toearly types in that they have large bulge fractions (median B/T = 0.59), highSersic index (n > 4), and high concentration indices (C > 4.3). The largefraction (70%) of E+As with positive color gradients (i.e., bluer nuclei)indicates that the young stellar populations are more concentrated than the oldpopulations. We show that these positive color gradients can evolve into thenegative gradients typical in E/S0s if the central parts of these galaxies aremetal enhanced. E+A galaxies stand apart from the E/S0s in the edge-onprojection of the Fundamental Plane, implying that E+As have, on average, a M/Lthat is ~3.8 times smaller than that of E/S0s. The tilt of the E+A FP indicatesthat the variation among stellar populations in these galaxies is closely tiedto their structural parameters such that smaller or less massive galaxies havesmaller M/L. We find a population of unresolved compact sources in nine E+As(45%). Their colors and luminosities are consistent with the hypothesis thatthese are newly formed star clusters. The bright end of the cluster LF isfainter in redder E+A's, suggesting that the young star cluster systems havefaded or been disrupted as the merger remnant aged. In summary, themorphologies, color profiles, scaling relations, and cluster populations areall consistent with the hypothesis that E+As galaxies are the results ofmergers that evolve into early-type galaxies.Comment: A typo fixed in the abstrac