Session 102 - Clusters of Galaxies.
Display session, Thursday, January 16
[102.06] Using Fundamental Plane Distances to Estimate the Total Binding Mass in Abell 2626
J. J. Mohr (U. Michigan), G. Wegner (Dartmouth Coll.)
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,
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listing for Thursday