The kinematics of the outer halo of M87 as mapped by planetary nebulae
  We present a kinematic study of a sample of 298 planetary nebulas (PNs) in
the outer halo of the central Virgo galaxy M87 (NGC~4486). The line-of-sight
velocities of these PNs are used to identify sub-components, to measure the
angular momentum content of the smooth M87 halo, and to constrain the orbital
distribution of the stars at these large radii. We find that the surface
density profile of the ICPNS at $100\, \mathrm{kpc}$ radii has a shallow
logarithmic slope, $-\alpha_{\rm ICL}\simeq -0.8$, dominating the light at the
largest radii. The angular momentum-related $\lambda_{\mathrm{R}}$ profile for
the smooth halo remains in the slow rotator regime, out to 135 kpc average
ellipse radius. The velocity dispersione profile is measured to decrease
sharply to $\sigma_{\mathrm{halo}} \sim 100\, \mathrm{kms}^{-1}$ at $R_{\rm
avg}=135$ kpc. Such a steeply decline of the velocity dispersion and the
surface density profile of the smooth halo can be reconciled with the circular
velocity curve inferred from assuming hydrostatic equilibrium for the hot X-ray
gas if the orbit distribution of the smooth M87 halo changes strongly from
approximately isotropic within $R_{\rm avg}\sim 60$ kpc to very radially
anisotropic at the largest distances probed. The extended LOSVD of the PNs in
the M87 halo allows the identification of several subcomponents: the ICPNs, the
"crown" accretion event, and the smooth M87 halo. In galaxies like M87, the
presence of these sub-components needs to be taken into account to avoid
systematic biases in estimating the total enclosed mass. The dynamical
structure inferred from the velocity dispersion profile indicates that the
smooth halo of M87 steepens beyond $R_{\rm avg}=60$ kpc and becomes strongly
radially anisotropic, and that the velocity dispersion profile is consistent
with the X-ray circular velocity curve at these radii without non-thermal
pressure effects.
