2011ApJ...729..129M

We present two-dimensional stellar kinematics of M87 out to R = 238'' taken with the integral field spectrograph VIRUS-P. We run a large set of axisymmetric, orbit-based dynamical models and find clear evidence for a massive dark matter halo. While a logarithmic parameterization for the dark matter halo is preferred, we do not constrain the dark matter scale radius for a Navarro-Frenk-White (NFW) profile and therefore cannot rule it out. Our best-fit logarithmic models return an enclosed dark matter fraction of 17.2^+5.0_–5.0% within one effective radius (R_e≅ 100''), rising to 49.4^+7.2_–8.8% within 2 R_e. Existing SAURON data (R ≤13''), and globular cluster (GC) kinematic data covering 145'' ≤ R ≤ 554'' complete the kinematic coverage to R = 47 kpc (∼5 R_e). At this radial distance, the logarithmic dark halo comprises 85.3^+2.5_–2.4% of the total enclosed mass of 5.7^+1.3_–0.9x10¹² M_☉making M87 one of the most massive galaxies in the local universe. Our best-fit logarithmic dynamical models return a stellar mass-to-light ratio (M/L) of 9.1^+0.2_–0.2(V band), a dark halo circular velocity of 800⁺⁷⁵_–25 km/s, and a dark halo scale radius of 36⁺⁷_–3kpc. The stellar M/L, assuming an NFW dark halo, is well constrained to 8.20^+0.05_–0.10(V band). The stars in M87 are found to be radially anisotropic out to R ≅0.5 R_e, then isotropic or slightly tangentially anisotropic to our last stellar data point at R = 2.4 R_e where the anisotropy of the stars and GCs are in excellent agreement. The GCs then become radially anisotropic in the last two modeling bins at R = 3.4 R_e and R = 4.8 R_e. As one of the most massive galaxies in the local universe, constraints on both the mass distribution of M87 and anisotropy of its kinematic components strongly inform our theories of early-type galaxy formation and evolution in dense environments.