GALAXY KINEMATICS WITH VIRUS-P: THE DARK MATTER HALO OF M87

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 (Re ≅ 100″), rising to 49.4+7.2−8.8% within 2 Re. Existing SAURON data (R ⩽13″), and globular cluster (GC) kinematic data covering 145″ ⩽ R ⩽ 554″ complete the kinematic coverage to R = 47 kpc (∼5 Re). At this radial distance, the logarithmic dark halo comprises 85.3+2.5−2.4% of the total enclosed mass of 5.7+1.3−0.9 × 1012 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-1, and a dark halo scale radius of 36+7−3 kpc. 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 Re, then isotropic or slightly tangentially anisotropic to our last stellar data point at R = 2.4 Re 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 Re and R = 4.8 Re. 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.