= Cluster velocity-dispersion mass estimate
{title2=$M\sim R\sigma_v^2/G$}
The <virial theorem> balances a relaxed <galaxy cluster>'s orbital <kinetic energy>, of order $M\sigma_v^2$, against its <Newtonian gravitational potential energy>, of order $-GM^2/R$. Their ratio gives the displayed mass scale. The numerical coefficient depends on the density profile, radius definition and conversion from line-of-sight <velocity dispersion> to three-dimensional orbital motion. Membership contamination, anisotropy and nonequilibrium dynamics can bias the inference.
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