The Dark Mass Concentration in the Central Parsec of the Milky Way


Max-Planck-Institut für extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany

THE ASTROPHYSICAL JOURNAL, 472:153-172, 1996 November 20


We report ~1" resolution K-band (2 µm) imaging spectroscopy of the central parsec of our Galaxy. The derived radial velocities for 223 early- and late-type stars probe the nuclear mass distribution to spatial scales of 0.1 pc. We find a statistically very significant increase of projected stellar velocity dispersion from about 55 km s-1 at p ~ 5 pc to 180 km s-1 at p ~ 0.1 pc. The stars are also rotating about the dynamic center. The late-type stars follow general Galactic rotation, while the early-type stars show counter-rotation. Fitting simultaneously the observed projected surface densities and velocity dispersions, we derive the intrinsic volume densities and radial velocity dispersions as a function of distance from the dynamic center for both types of stars. We then derive the mass distribution between 0.1 and 5 pc from the Jeans equation assuming an isotropic velocity Field. Our analysis requires a compact central dark mass of 2.5-3.2×106 at 6-8 sigma significance. The dark mass has a density of 109 Mopc-3 or greater and a mass to 2 µm luminosity of >100. The increase in mass-to-luminosity ratio can be reduced but not eliminated even if extreme anisotropic velocity destributions are considered. The dark mass cannot be a cluster of solar mass remnants (such as neutron stars). It is either a compact cluster of 10-20 Mo black holes or a single massive black hole.

Subject headings: celestial mechanics, stellar dynamics - Galaxy: center - infrared : stars - techniques : radial velocities 

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