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The Fundamental Plane of Galaxy Clusters
Velocity dispersion , radius and luminosity of elliptical
galaxies are known to be related, leaving only two degrees of freedom and
defining the so-called ``fundamental plane". In this {\em Letter} we present
observational evidence that rich galaxy clusters exhibit a similar behaviour.
Assuming a relation , the best-fit values
of and are very close to those defined by galaxies. The
dispersion of this relation is lower than 10 percent, i.e. significantly
smaller than the dispersion observed in the and relations. We
briefly suggest some possible implications on the spread of formation times of
objects and on peculiar velocities of galaxy clusters.Comment: 11pp., 4 figures (available on request), LaTeX, BAP-04-1993-015-OA
Multiple merging events in Abell 521
We present a detailed spatial and dynamical analysis of the central
~2.2~\h~Mpc region of the galaxy cluster Abell~521 (z=0.247), based on
238 spectra obtained at the 3.6~m Telescope of ESO and at the CFHT. From the
analysis of the 125 galaxies confirmed members of the cluster, we derive a mean
velocity of km/s and detect a complex velocity
distribution with high velocity dispersion, km/s), but
clear departure from a single gaussian component. The general structure of the
cluster follows a NW/SE direction, crossed by a perpendicular high density
``ridge'' of galaxies in the core region. The northern region of the cluster is
characterized by a lower velocity dispersion as compared to the whole cluster
value; it hosts the BCG and a dynamically bound complex of galaxies, and it is
associated to a group detected in X-ray (Arnaud et al 2000). This region could
be in pre-merger stage onto the main cluster nearly in the plane of the sky.
These results, taken together with the fact that most of the clumps detected on
the isodensity maps, as well as the early type galaxies and the brightest ones
are aligned, suggest that this NW/SE direction is the preferred one for the
formation of this cluster. The central high dense region shows a lower velocity
location ( km/s) and significantly higher scale ( km/s) as compared to the whole cluster values. This is due to
the presence of a low-velocity group of galaxies with a high fraction of
emission line objects. This can be explained in a scenario in which a merging
of subclusters has recently occurred along the direction of the ``ridge'' with
a significant component along the line of sight.Comment: 21 pages, 32 figures, uses aa.cls style, Latex. Accepted for
publication in A&
High-Order Correlations of Rich Galaxy Clusters
We analyse the two--dimensional all--sky distribution of rich Abell and ACO
galaxy clusters by using counts in cells and measuring the high--order
area--averaged angular correlation functions. Confirming previous results, we
find a well defined hierarchical relation between the two and three--point
correlation functions, remarkably constant with scale. In the angular range
, the southern sample, limited at and including both Abell and ACO clusters, shows a remarkable
hierarchical behavior up to the 6th order, while northern Abell clusters give
positive correlations in the same range only up to the 4th order. The inferred
deprojected values of the 3--D coefficients , where , are similar to those measured for the galaxy
distribution, and consistent with theoretical predictions. These results are
confirmed to the 4th order by our analysis of a 3--D sample of Abell and ACO
clusters. Assuming that selection effects and / or the absence of a cluster
fair sample are the reason of the difference between the two galactic
hemispheres, and between Abell and ACO clusters, our results indicate that the
statistical properties of the cluster distribution originate from the
underlying galaxy distribution and show that the biasing between clusters and
galaxies is non--linear.Comment: 29 pp., (ApJ, accepted for publication). This is a uuencoded
compressed postscript file including figure
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