The Origin of E+a Galaxies

1. E+A Mystery E+A galaxies are galaxies with strong Balmer absorption lines and with no significant emission in [OII] or Hα lines. The existence of strong Balmer absorption lines shows that these galaxies have experienced a starburst within the last 1 Gyr. At the same time, these galaxies do not show any sign of ongoing star formation as indicated by the lack of [OII] or Hα emission lines. Therefore, E+A galaxies are interpreted as a post-starburst galaxy — a galaxy which truncated starburst suddenly. However, for more than 20 years, the origin of this post-starburst phenomena remained a mystery. Using the 100,000 galaxy spectra of the Sloan Digital Sky Survey, we tested three popular explanations to the E+A phenomenon. 2. Are E+As Created by a Cluster-Related Mechanism? At first, E+A galaxies were found in cluster regions. Therefore, cluster related physical mechanisms have been thought to be responsible for the E+A phenomena. However, in the left panel of Fig. 1, E+A galaxies show the same local galaxy density distribution as field (all) galaxies. These numerous field E+A galaxies cannot be explained with a cluster related physical mechanism. 3. Are E+As Dusty Starbursts? Another possible explanation for E+A phenomena is a dusty starburst, where E+A galaxies are actually star-forming, but emission lines are invisible in optical wavelengths due to the heavy obscuration by dust. However, in the middle panel of Fig.1, all E+As have r − K < 1.1, where the theoretical models predict r − K ∼ 1.5 for dusty starburst galaxies. In the left panel of Fig.1, no E+As show any sign of starburst in radio estimated (dust-free) star formation rate derived from the FIRST data.