The Fundamental Plane in Rx J0142.0+2131: a Galaxy Cluster Merger at Z = 0.28

We present the Fundamental Plane (FP) in the z = 0.28 cluster of galaxies RX J0142.0+2131. There is no evidence for a difference in the slope of the FP when compared with the Coma cluster, although the internal scatter is larger. On average, stellar populations in RX J0142.0+2131 have rest-frame V -band mass-to-light ratios (M/LV ) 0.29± 0.03 dex lower than in Coma. This is significantly lower than expected for a passivelyevolving cluster formed at zf = 2. Lenticular galaxies have lower average M/LV and a distribution of M/LV with larger scatter than ellipticals. Lower mass-to-light ratios are not due to recent star formation: our previous spectroscopic observations of RX J0142.0+2131 E/S0 galaxies showed no evidence for significant star-formation within the past ∼ 4 Gyr. However, cluster members have enhanced α-element abundance ratios, which may act to decrease M/LV . The increased scatter in the RX J0142.0+2131 FP reflects a large scatter in M/LV implying that galaxies have undergone bursts of star formation over a range of epochs. The seven easternmost cluster galaxies, including the second brightest member, have M/LV consistent with passive evolution and zf = 2. We speculate that RX J0142.0+2131 is a cluster–cluster merger where the galaxies to the east are yet to fall into the main cluster body or have not experienced star formation as a result of the merger. Subject headings: galaxies: clusters: individual: RX J0142.0+2131 – galaxies: evolution – galaxies: stellar content