Star formation in galaxies along the Pisces-Cetus Supercluster filaments

We investigate the variation of current star formation in galaxies as a function of distance along three supercluster filaments, each joining pairs of rich clusters, in the Pisces-Cetus supercluster, which is part of the two-degree Field Galaxy Redshift Survey (2dFGRS). We find that even though there is a steady decline in the rate of star formation, as well as in the fraction of star forming galaxies, as one approaches the core of a cluster at an extremity of such a filament, there is an increased activity of star formation in a narrow distance range between 3-4 h 70 Mpc, which is 1.5–2 times the virial radius of the clusters involved. This peak in star formation is seen to be entirely due to the dwarf galaxies (−20<MB 6−17.5). The position of the peak does not seem to depend on the velocity dispersion of the nearest cluster, undermining the importance of the gravitational effect of the clusters involved. We find that this enhancement in star formation occurs at the same place for galaxies which belong to groups within these filaments, while group members elsewhere in the 2dFGRS do not show this effect. We conclude that the most likely mechanism for this enhanced star formation is galaxy-galaxy harassment, in the crowded infall region of rich clusters at the extremities of filaments, which induces a burst of star formation in galaxies, before they have been stripped of their gas in the denser cores of clusters. The effects of strangulation in the cores of clusters, as well as excess star formation in the infall regions along the filaments, are more pronounced in dwarfs since they more vulnerable to the effects of strangulation and harassment than giant galaxies.