  We analyse the properties of galaxy populations in the rich Abell cluster
ABCG 209 at redshift z~0.21, on the basis of spectral classification of 102
member galaxies. We take advantage of available structural parameters to study
separately the properties of bulge-dominated and disk-dominated galaxies. The
star formation histories of the cluster galaxy populations are investigated by
using line strengths and the 4000 A break, through a comparison to stellar
population synthesis models. The dynamical properties of different spectral
classes are examined in order to infer the past merging history of ABCG 209.
The cluster is characterized by the presence of two components: an old galaxy
population, formed very early (z_f >~ 3.5), and a younger (z$_f >~ $ 1.2)
population of infalling galaxies. We find evidence of a merger with an
infalling group of galaxies occurred 3.5-4.5 Gyr ago. The correlation between
the position of the young H_delta-strong galaxies and the X-ray flux shows that
the hot intracluster medium triggered a starburst in this galaxy population ~ 3
Gyr ago.

Astrophysics