Abundance diagnosis of E+A (post-starburst) galaxies

Abstract

E+A galaxies are characterized as a galaxy with strong Balmer absorption lines but without any [OII] or H$\alpha$ emission lines. The existence of strong Balmer absorption lines means that E+A galaxies have experienced a starburst within the last $<$1-1.5 gigayear. However, the lack of [OII] or H$\alpha$ emission lines indicates that E+A galaxies do not have any on-going star formation. Therefore, E+A galaxies are interpreted as a post-starburst galaxy. Morphologically, E+A galaxies appear as early-type galaxies, implying E+A galaxies may be one of the progenitors of present-day elliptical galaxies. However, there remained other possibilities such as the dusty starburst scenario, where E+A galaxies have on-going star formation, but optical emission lines are invisible due to the heavy obscuration by dust. Therefore, an additional evidence of the post-starburst phenomena has been eagerly awaited. Using one of the largest samples of 451 E+A galaxies carefully selected from the Sloan Digital Sky Survey Data Release 4, here we show the abundance diagnosis of E+A galaxies using Mg and Fe lines. Our findings are as follows :(i) E+A galaxies has enhanced $\alpha$-element abundance ratio compared to the star-forming galaxies with similar Balmer absorption strength. Since the truncation of strong starburst is required to enhance the alpha element ratio, this is an additional evidence that E+A galaxies are in the post-starburst phase; (ii) the metallicity and $\alpha$-element abundance of E+A galaxies are consistent with those of elliptical galaxies, suggesting that E+A galaxies could be one of the progenitors of present-day elliptical galaxies in terms of chemical abundances.