Integrated star formation rate ( SFR ) and specific star formation rate ( SFR/ M ^ { * } ) , derived from the spectroscopic data obtained by SDSS DR4 are used to show that the star formation activity in galaxies ( M _ { r } \leq - 20.5 ) found on the outskirts ( 1-2r _ { 200 } ) of some nearby clusters ( 0.02 \leq z \leq 0.15 ) is enhanced .
By comparing the mean SFR of galaxies in a sample of clusters with at least one starburst galaxy ( log SFR/ M ^ { * } \geq - 10 yr ^ { -1 } and SFR \geq 10 M _ { \odot } yr ^ { -1 } ) 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 .
Compared to the clusters with at least one starburst galaxy on their outskirts , the galaxies in the ‘ comparison ’ clusters show a low mean SFR at all radius ( \leq 3 r _ { 200 } ) from the cluster centre .
Such an increase in the SFR of galaxies is more likely to be seen in dynamically unrelaxed ( \sigma _ { v } \gtrsim 500 km s ^ { -1 } ) 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 .