Plunging fireworks: Why do infalling galaxies light up on the outskirts of clusters?

TL;DR

This study shows that galaxies on the outskirts of certain clusters exhibit enhanced star formation activity, especially in unrelaxed clusters with high velocity dispersions, due to galaxy interactions in dense infalling regions.

Contribution

It reveals the environmental dependence of star formation enhancement in infalling galaxies and links it to cluster dynamical state and filamentary accretion processes.

Findings

Star formation rate is higher in outskirts of some clusters.

Unrelaxed clusters show more starburst activity.

Galaxy interactions in dense infalling regions trigger starbursts.

Abstract

(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.