  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.
