The impact of a major cluster merger on galaxy evolution in MACS J0025.4−1225

Abstract

We present results of an extensive morphological, spectroscopic, and photometric study of the galaxy population of MACS J0025.4$-$1225 (z=0.586), a major cluster merger with clear segregation of dark and luminous matter, to examine the impact of mergers on galaxy evolution. Based on 436 galaxy spectra obtained with Keck DEIMOS, we identified 212 cluster members within 4 Mpc of the cluster centre, and classified them using three spectroscopic types; we find 111 absorption-line, 90 emission-line (including 23 e(a) and 11 e(b)), and 6 E+A galaxies. The fraction of absorption(emission)-line galaxies is a monotonically increasing(decreasing) function of both projected galaxy density and radial distance to the cluster center. More importantly, the 6 observed E+A cluster members are all located between the dark-matter peaks of the cluster and within ~0.3Mpc radius of the X-ray flux peak, unlike the E+A galaxies in other intermediate-redshift clusters which are usually found to avoid the core region. In addition, we use Hubble Space Telescope imaging to classify cluster members according to morphological type. We find the global fraction of spiral and lenticular galaxies in MACS J0025 to be among the highest observed to date in clusters at z>0.5. The observed E+A galaxies are found to be of lenticular type with Sersic indices of ~2, boosting the local fraction of S0 to 70 per cent between the dark-matter peaks. Combing the results of our analysis of the spatial distribution, morphology, and spectroscopic features of the galaxy population, we propose that the starburst phase of these E+A galaxies was both initiated and terminated during the first core-passage about 0.5--1Gyr ago, and that their morphology has already been transformed into S0 due to ram pressure and/or tidal forces near the cluster core. By contrast, ongoing starbursts are observed predominantly in infalling galaxies, and thus appears to be unrelated to the cluster merger.