Evidence in Virgo for the Universal Dark Matter Halo

A model is constructed for the mass and dynamics of M87 and the Virgo Cluster. Existing surface photometry of the galaxy, mass estimates from X-ray observations of the hot intracluster gas, and the velocity dispersions of early-type Virgo galaxies are all used to constrain the run of dark matter density over 0 ≤ r ≲ 2 Mpc in the cluster. The "universal" halo advocated by Navarro, Frenk, & White, ρ dm ∝ ( r / r s ) -1 (1 + r / r s ) -1 , provides an excellent description of the combined data, as does a Hernquist profile with ρ dm ∝ ( r / r s ) -1 (1+ r / r s ) -3 . These models are favored over isothermal spheres, and their central structure is preferred to density cusps either much stronger or much weaker than r -1 . The galaxies and gas in the cluster trace its total mass distribution (ρ gal ∝ ρ gas ∝ ρ dm ), the galaxies' velocity ellipsoid is close to isotropic, and the gas temperature follows the virial temperature profile of the dark halo. The virial radius and mass and the intracluster gas fraction of Virgo are evaluated.