266 E+A Galaxies Selected from the Sloan Digital Sky Survey Data Release 2: The Origin of E+A Galaxies

E+A galaxies are characterized as a galaxy with strong Balmer absorption lines but without any [OII] nor H$\alpha$ emission lines. The existence of strong Balmer absorption lines indicates that E+A galaxies have experienced starburst within recent one Gyr. However, the lack of [OII] and 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. For many years, however, it has been a mystery why E+A galaxies started starburst and why they quenched the star formation abruptly. Using one of the largest samples of 266 E+A galaxies carefully selected from the Sloan Digital Sky Survey Data Release 2, we have investigated the environment of E+A galaxies from 50 kpc to 8 Mpc scale, i.e., from a typical distance to satellite galaxies to the scale of large scale structures. We found that E+A galaxies have excess of local galaxy density only at a scale of $<100$ kpc (with a two $\sigma$ significance), but not at the cluster scale ($\sim$1.5 Mpc) nor in the scale of large scale structure ($\sim$8 Mpc). These results indicate that E+A galaxies are not created by the physical mechanisms associated with galaxy clusters or the large scale structure, but are likely to be created by the dynamical interaction with closely accompanying galaxies at a $<$100 kpc scale. The claim is also supported by the morphology of E+A galaxies. We have found that almost all E+A galaxies have a bright compact core, and that $\sim$30% of E+A galaxies have dynamically disturbed signatures or the tidal tails, being quite suggestive of morphological appearance of merger/interaction remnants.