Interacting galaxies in the A901/902 supercluster with STAGES

We present a study of galaxy mergers and the influence of environment in the Abell 901/902 supercluster at z ∼ 0.165, based on 893 bright (R_Vega≤24) intermediate-mass (M_∗ ≥10⁹M) galaxies. We use HST ACS F606W data from the Space Telescope A901/902 Galaxy Evolution Survey, COMBO-17, Spitzer 24μm, and XMM-Newton X-ray data. Our analysis utilizes both a physically driven visual classification system and quantitative CAS parameters to identify systems which show evidence of a recent or ongoing merger of mass ratio >1/10 (i.e., major and minor mergers). Our results are (1) after visual classification and minimizing the contamination from false projection pairs, we find that the merger fraction f_merge is 0.023 ± 0.007. The estimated fractions of likely major mergers, likely minor mergers, and ambiguous cases are 0.01 ± 0.004, 0.006 ± 0.003, and 0.007 ± 0.003, respectively. (2) All the mergers lie outside the cluster core of radius R < 0.25 Mpc: the lack of mergers in the core is likely due to the large galaxy velocity dispersion in the core. The mergers, instead, populate the region (0.25 Mpc < R 2 Mpc) between the core and the cluster outskirt. In this region, the estimated frequency of mergers is similar to those seen at typical group overdensities in N-body simulations of accreting groups in the A901/902 clusters. This suggests the ongoing growth of the clusters via accretion of group and field galaxies. (3) We compare our observed merger fraction with those reported in other clusters and groups out to z ∼ 0.4. Existing data points on the merger fraction for L ≤L^∗ galaxies in clusters allow for a wide spectrum of scenarios, ranging from no evolution to evolution by a factor of ∼5 over z ∼ 0.17–0.4. (4) In A901/902, the fraction of interacting galaxies, which lie on the blue cloud is 80% ± 18% (16/20) versus 34% ± 7% or (294/866) for non-interacting galaxies, implying that interacting galaxies are preferentially blue. (5) The average star formation rate (SFR), based on UV or a combination of UV+IR data, is enhanced by a factor of ∼1.5–2 in mergers compared to non-interacting galaxies. However, mergers in the A901/902 clusters contribute only a small fraction (between 10% and 15%) of the total SFR density, while the rest of the SFR density comes from non-interacting galaxies.