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We model the dynamical structure of M87 (NGC4486) using high spatialresolution 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 fitfor four parameters, black hole mass, dark halo core radius, dark halo circularvelocity, and stellar mass-to-light ratio. We find a black hole mass of6.4(+-0.5)x10^9 Msun(the uncertainty is 68% confidence marginalized over theother parameters). The stellar M/L_V=6.3+-0.8. The best-fitted dark halo coreradius is 14+-2 kpc, assuming a cored logarithmic potential. The best-fitteddark halo circular velocity is 715+-15 km/s. Our black hole mass is over afactor of 2 larger than previous stellar dynamical measures, and our derivedstellar M/L ratio is 2 times lower than previous dynamical measures. When we donot include a dark halo, we measure a black hole mass and stellar M/L ratiothat is consistent with previous measures, implying that the major differenceis in the model assumptions. The stellar M/L ratio from our models is verysimilar to that derived from stellar population models of M87. The reason forthe difference in the black hole mass is because we allow the M/L ratio tochange with radius. The dark halo is degenerate with the stellar M/L ratio,which is subsequently degenerate with the black hole mass. We argue thatdynamical models of galaxies that do not include the contribution from a darkhalo may produce a biased result for the black hole mass. This bias isespecially large for a galaxy with a shallow light profile such as M87, and maynot be as severe in galaxies with steeper light profiles unless they have alarge stellar population change with radius.

The Black Hole Mass, Stellar M/L, and Dark Halo in M87