  We model the dynamical structure of M87 (NGC4486) using high spatial
resolution long-slit observations of stellar light in the central regions,
two-dimensional stellar light kinematics out to half of the effective radius,
and globular cluster velocities out to 8 effective radii. We simultaneously fit
for four parameters, black hole mass, dark halo core radius, dark halo circular
velocity, and stellar mass-to-light ratio. We find a black hole mass of
6.4(+-0.5)x10^9 Msun(the uncertainty is 68% confidence marginalized over the
other parameters). The stellar M/L_V=6.3+-0.8. The best-fitted dark halo core
radius is 14+-2 kpc, assuming a cored logarithmic potential. The best-fitted
dark halo circular velocity is 715+-15 km/s. Our black hole mass is over a
factor of 2 larger than previous stellar dynamical measures, and our derived
stellar M/L ratio is 2 times lower than previous dynamical measures. When we do
not include a dark halo, we measure a black hole mass and stellar M/L ratio
that is consistent with previous measures, implying that the major difference
is in the model assumptions. The stellar M/L ratio from our models is very
similar to that derived from stellar population models of M87. The reason for
the difference in the black hole mass is because we allow the M/L ratio to
change with radius. The dark halo is degenerate with the stellar M/L ratio,
which is subsequently degenerate with the black hole mass. We argue that
dynamical models of galaxies that do not include the contribution from a dark
halo may produce a biased result for the black hole mass. This bias is
especially large for a galaxy with a shallow light profile such as M87, and may
not be as severe in galaxies with steeper light profiles unless they have a
large stellar population change with radius.
