Plunging fireworks: Why do infalling galaxies light up on the outskirts
  of clusters?
  (abridged)Integrated star formation rate (SFR) and specific star formation
rate (sSFR), derived from the spectroscopic data obtained by SDSS DR4 are used
to show that the star formation activity in galaxies (M_r<=-20.5) found on the
outskirts (1-2r_{200}) of some nearby clusters (0.02<=z<=0.15) is enhanced. By
comparing the mean SFR of galaxies in a sample of clusters with at least one
starburst galaxy (log sSFR>=-10/yr & SFR>10 M_sun/yr) to a sample of clusters
without such galaxies (`comparison' clusters), we find that despite the
expected decline in the mean SFR of galaxies toward the cluster core, the SFR
profile of the two samples is different, such that the galaxies in the
`comparison' clusters show a lower mean SFR at all radius (<=3r_{200}) from the
cluster centre. Such an increase in the SFR of galaxies is more likely to be
seen in dynamically unrelaxed (sigma_v>~500 km/s) clusters. It is also evident
that these unrelaxed clusters are currently being assembled via galaxies
falling in through straight filaments, resulting in high velocity dispersions.
On the other hand, `comparison' clusters are more likely to be fed by
relatively low density filaments. We find that the starburst galaxies on the
periphery of clusters are in an environment of higher local density than other
cluster galaxies at similar radial distances from the cluster centre. We
conclude that a relatively high galaxy density in the infalling regions of
clusters promotes interactions amongst galaxies, leading to momentary bursts of
star formation. Such interactions play a crucial role in exhausting the fuel
for star formation in a galaxy, before it is expelled due to the environmental
processes that are operational in the dense interiors of the cluster.
