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The Astronomical Journal


We use fundamental plane (FP) distance estimates to the components of the double cluster A2626 (cz ~ 17,500 km/s) to constrain cluster kinematics and estimate total binding mass. We employ deep R band CCD photometry, multi-object spectroscopy, and software designed to account for seeing effects to measure the FP parameters R_e, sigma , and for 24 known early type and S0 cluster members. The FP coefficients from this sample (alpha =1.30+/-0.36 and beta =0.31+/-0.06) are consistent with others reported in the literature. We examine the Mg_b equivalent width distributions within both subclusters and find them to be indistinguishable. Lacking evidence for stellar population differences, we interpret the FP zeropoint offset between the two subclusters as a measure of the distance difference. We find log (D_B/D_A)=-0.037+/-0.046, where Dcl is the distance to subcluster cl. This measurement is consistent with the subclusters being at the same distance, and it rules out the Hubble flow hypothesis (distances proportional to velocity) with 99% confidence; analysis of the subcluster galaxy magnitude distributions rules out Hubble flow at 93% confidence. Both results favor a kinematic model where the subclusters are bound and infalling. We estimate the total cluster binding mass by modelling the subcluster merger as radial infall. The projected separation, the line of sight velocity difference and the line of sight separation constrain the cluster mass; the minimum possible total binding mass is 1.65 times higher than the sum of the standard virial masses, a difference statistically significant at the ~ 3sigma level. We discuss explanations for the inconsistency including (1) biases in the standard virial mass estimator, (2) biases in our radial infall mass estimate, and (3) mass beyond the virialized cluster region; if the standard virial mass is significantly in error, the cluster has an unusually high mass-to-light ratio ( ~ 1000h). Because observational signatures of departures from radial infall are absent, we explore the implications of mass beyond the virialized, core regions.