  We have investigated the effect of pressure from intracluster medium (ICM) on
disk galaxies in merging clusters. The ram-pressure on the galaxies rapidly
increases when two clusters collide. This leads to stripping of the
interstellar medium (ISM) and decrease of star formation rate (SFR) of the
galaxies. On the other hand, the increase of SFR caused by compression of ISM
is less significant. Thus, cluster merger does not trigger, but weakens star
formation activity of the galaxies. In the central region of the colliding
clusters, blue galaxies with high velocity should exist, although most of
galaxies become red. Following the decrease of blue galaxy fraction in the
clusters, the fraction of post-starburst galaxies increases. After merger, many
galaxies in the cluster restart star formation activity and the segregation of
blue and red galaxies becomes prominent.

Astrophysics