Evidence for galaxies being pre-processed before accreted into clusters

Abstract

I use the spectroscopic data for galaxies in and around nearby (0.02<z<0.06) X-ray bright galaxy clusters, to show that the incidence of k+a (or post-starburst) galaxies (EW(Ha)<2 ang in emission and EW(Hd)>3 ang in absorption) may be correlated with the accretion of small galaxy groups in clusters. At r<2r200, the k+a galaxies are found in regions of higher galaxy density relative to other cluster galaxies. The k+a galaxies have a positively skewed distribution of absolute velocity,|v_los|/sigma_v, where v_los is the difference between the line-of-sight velocity of the galaxy and the cluster's mean, and sigma_v is the cluster's velocity dispersion. This distribution is statistically different from that of other cluster galaxies within 2r200, and in the same absolute velocity range. Moreover, 87% of clusters in the sample studied here show statistically significant substructure in their velocity distribution, and 91.4% of all k+a galaxies are found to be a part of one of these substructures with 4-10 members. These results suggest that star formation in these k+a galaxies is likely to have been quenched due to "pre-processing" in a poor group-like environment before they are accreted into clusters. I also find a mild, but statistically significant trend in the fraction of k+a galaxies increasing with the temperature of the X-ray emitting gas in clusters.