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 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 11 ( 55 % ) 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 \gtrsim 4 ) , and high concentration indices ( C \gtrsim 4.3 ) , but they have considerably larger asymmetry indices ( A \gtrsim 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+A ’ s , 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 .