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 galaxies with strong Balmer absorption lines but without any [OII] or Halpha emission lines. The existence of strong Balmer absorption lines indicates that E + A galaxies have experienced starburst within the past one gigayear. However, the lack of [OII] and H emission lines indicates that E + A galaxies do not have any on-going star formation. Therefore, E + A galaxies are interpreted as post-starburst galaxies. For many years, however, it has been a mystery why E + A galaxies started starburst and why they quenched 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 an excess of local galaxy density only at a scale of < 100 kpc (with a 2 significance), but not at the cluster scale ( 1.5 Mpc) nor at the scale of large-scale structure ( 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 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 30 per cent of E + A galaxies have dynamically disturbed signatures or tidal tails, which quite strongly suggest the morphological appearance of merger/interaction remnants.