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Session 102 - Clusters of Galaxies.
Display session, Thursday, January 16
Metropolitan Ballroom,

[102.06] Using Fundamental Plane Distances to Estimate the Total Binding Mass in Abell 2626

We use fundamental plane (FP) distance estimates to the components of the double cluster A2626 (cz\sim17,500 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 extract the FP parameters R_e, \sigma, and \left<\mu_e\right> for 25 known cluster members. Spectroscopy of 13 galaxies yields dispersions accurate to better than 20%; the FP coefficients from this sample are consistent with determinations in the literature. We detect a significant, \sim0.03 mag offset in the Mg_2--\log\sigma relations for the two subclusters; the offset is consistent with the known correlation between cluster velocity dispersion and \left. We explore the possibility of M/L_R zeropoint differences in the two clusters before using the FP zeropoint offset to constrain the relative distances to the two subclusters. The distance constraint is \log(D_B/D_A)=-0.078\pm0.064, where D_cl is the distance to subcluster cl. This rules out Hubble flow (\log(D_B/D_A)=0.065) at 2.2\sigma (97% confidence); an 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. Modelling the cluster merger as a radial infall and using the observables, we estimate the total binding mass. Specifically, 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 a factor of 1.65 higher than the sum of the standard virial masses, a difference statistically significant at the \sim3\sigma level. We discuss explanations for the inconsistency including biases in the standard virial mass, (2) biases in the radial infall mass, 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 (\sim1000h). Because observational signatures of departures from radial infall are absent, we explore the implications of mass beyond the virialized, core regions.

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