The Detailed Evolution of E+A Galaxies into Early Types

Abstract

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 HSTACS and WFPC2 images. Most of these galaxies lie in the field, well outside of rich clus- ters, 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 inthattheyhavelargebulgeYtoYtotallightratios(medianB/T ¼ 0:59),highSersicindices(nk4),andhighconcen- trationindices(Ck4:3),buttheyhaveconsiderablylargerasymmetry indices(Ak0:04)thanellipticals,presumably due to the disturbances within a fewre caused by the starburst and/or the galaxy-galaxy interaction. We conclude that E+Aswillbemorphologicallyclassifiedasearly-typegalaxiesoncethesedisturbancesandthelowsurfacebrightness tidal features fade. The color morphologies are diverse, including six E+As with compact (0.4Y1.4 kpc) blue cores, which might be local analogs of high-z ellipticals with blue cores. The large fraction (70%) of E+As with positive colorgradientsindicatesthattheyoungstellarpopulationsaremoreconcentratedthantheold.Thesepositivecolor 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. Wefind a population of unresolved compact sources in nineE+As (45%), all of which havemerger 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. Subject headingg galaxies: evolution — galaxies: interactions — galaxies: starburst — galaxies: star clusters — galaxies: stellar content