2008ApJ...688..945Y

Poststarburst, 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 as a result of galaxy-galaxy interactions. To focus on what E+A galaxies become after their young stellar populations fade away, we present the detailed morphologies of 21 E+A galaxies using high-resolution HST ACS and WFPC2 images. Most of these galaxies lie in the field, well outside of rich clusters, and at least 55 (15%) have dramatic tidal features indicative of mergers. Our sample includes one binary E+A system, in which both E+As are tidally disturbed and interacting with each other. Our E+As are similar to early types in that they have large bulge-to-total light ratios (median B/T=0.59), high Sérsic indices (n≳4), and high concentration indices (C≳4.3), but they have considerably larger asymmetry indices (A≳0.04) than ellipticals, presumably due to the disturbances within a few r_e caused by the starburst and/or the galaxy-galaxy interaction. We conclude that E+As will be morphologically classified as early-type galaxies once these disturbances and the low surface brightness tidal features fade. The color morphologies are diverse, including six E+As with compact (0.4-1.4 kpc) blue cores, which might be local analogs of high-z ellipticals with blue cores. The large fraction (70%) of E+As with positive color gradients indicates that the young stellar populations are more concentrated than the old. These positive color gradients (i.e., bluer nuclei) could evolve into the negative gradients typical in E/S0s if the central parts of these galaxies are metal-enhanced. Our E+As stand apart from the E/S0s in the edge-on projection of the fundamental plane (FP), implying that their stellar populations differ from those of E/S0s and that E+As have, on average, a M/L that is 3.8 times smaller. The tilt of the E+A FP indicates that the variation among their stellar populations is closely tied to the structural parameters; i.e., E+As follow their own scaling relationships such that smaller or less massive galaxies have smaller M/L. We find a population of unresolved compact sources in nine E+As (45%), all of which have merger signatures. In the four E+As with suitable color data, the compact sources have colors and luminosities consistent with newly formed star clusters. The bright end of the cluster LF is fainter in redder E+As, suggesting that the young star clusters fade or are disrupted as the merger remnant ages. In summary, the morphologies, color profiles, scaling relations, and cluster populations are all consistent with E+As evolving ultimately into early types, making the study of E+As critical to understanding the origin of the red sequence of galaxies.