Oral, Tuesday, November 7, 2000, 1:00-2:30pm, Pago Pago Ballroom

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*A.E. Evrard (University of Michigan, Departments of Physics & Astronomy), T. Horikawa (Osaka University, X-ray Astronomy Group), Virgo Consortium Collaboration*

The Hubble Volume project of the Virgo Consortium has
created 10^{9} particle N-body simulations of large-scale
structure formation in {\Lambda}CDM and {\tau}CDM
cosmologies with resolution sufficient to define a virtual
Coma cluster with 500 particles. Light-cone survey output
from the simulations provide synthetic sky surveys of the
dark matter distribution in very large cosmic volumes, ~
10^{10} h^{-3} Mpc^{3}. Cluster catalogs derived from the
surveys contain 100,000 to 500,000 clusters with masses
exceeding 5 \times 10^{13} h^{-1} {\rm M}_{\}odot and
redshifts extending to z ~2.

We analyse in detail the virial relation between dark matter
mass M_{\Delta_c} and velocity dispersion \sigma. We
find a unified calibration of the relation in the form H(z)
M_{\Delta_c} = A \sigma^{p} for which the amplitude A and
slope p are independent of cosmology and/or epoch (H(z)
is the Hubble parameter at redshift z). This holds for
clusters whose properties are defined within a spherical
region encompassing a fixed density contrast \Delta_{c}
(typically 200) with respect to the critical density. Other
definitions of clusters require a redshift dependent
amplitude A(z). The scatter in \sigma at fixed H(z) M
about the mean relation is small (~6%) and positively
skewed. Subdividing the population into two classes ---
`parents' and `children' --- we identify the minority child
component as the source of the skewness and infer that the
children are merger debris that has not yet been fully
incorporated into the parent population. For the parents
alone, the probability distribution function of the velocity
dispersion residuals is very well modeled by a Gaussian
distribution, suggesting a central limit theorem
interpretation.

The accuracy of the calibration will be addressed by examining Virgo simulations with higher mass resolution and smaller volumes. Connections to obervable measures --- cluster X-ray temperature and galaxy velocity dispersion --- will be briefly discussed.

If you would like more information about this abstract, please follow the link to http://www.physics.lsa.umich.edu/hubble-volume. This link was provided by the author. When you follow it, you will leave the Web site for this meeting; to return, you should use the Back comand on your browser.

The author(s) of this abstract have provided an email address for comments about the abstract: evrard@umich.edu

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