Stellar Kinematics and the Black Hole in the Galactic Center

We estimate the amount and distribution of matter in the Galactic center, based on measurements of stellar velocities. In addition to published data, we consider new observations of the CO (ν = 2 ← 0) 2.3 microns absorption feature in the unresolved stellar emission at 41 positions within 20" of Sgr A^*. Because the CO band strength is greatly reduced within 6"-8" of Sgr A^*, we model the three-dimensional distribution of the stars with this feature. We find that the CO-bearing stars can provide a useful lower limit to the enclosed mass. We also analyze the velocities of the He I-emitting stars within 20" of Sgr A^*. We find a correlation between the He I line width and the radial velocity of these stars, indicating that the velocity dispersion derived from the He I stars is an upper limit to the intrinsic value, and hence the corresponding enclosed mass is an upper limit. Because this upper limit and the lower limits derived from the CO-bearing stars are virtually the same, at just under 2 × 10⁶ M_sun, we conclude that the stellar dynamics require a concentrated central mass of this amount. This value is in close agreement with the mass deduced from gas velocities measured with the [Ne II] line. Although it is most likely that this central mass is in the form of a black hole, we cannot exclude the possibility of a tightly concentrated cluster of stellar remnants. In addition, we infer the possible existence of an extended component of dark matter, which might also be attributed to stellar remnants.