The role of E+A and post-starburst galaxies - I. Models and model
  results
  Different compositions of galaxy types in the field in comparison to galaxy
clusters as described by the morphology-density relation in the local universe
is interpreted as a result of transformation processes from late- to early-type
galaxies. This interpretation is supported by the Butcher-Oemler effect.
  We investigate E+A galaxies as an intermediate state between late-type
galaxies in low density environments and early-type galaxies in high density
environment to constrain the possible transformation processes. For this
purpose we model a grid of post-starburst galaxies by inducing a burst and/or a
halting of star formation on the normal evolution of spiral galaxies with our
galaxy evolution code GALEV.
  From our models we find that the common E+A criteria exclude a significant
number of post-starburst galaxies and propose that comparing their spectral
energy distributions leads to a more sufficient method to investigate
post-starbust galaxies. We predict that a higher number of E+A galaxies in the
early universe can not be ascribed solely to a higher number of starburst, but
is a result of a lower metallicity and a higher burst strength due to more gas
content of the galaxies in the early universe. We find that even galaxies with
a normal evolution without a starburst have a Hdelta-strong phase at early
galaxy ages.
