VELOCITY DISPERSION AND THE STELLAR POPULATION IN THE CENTRAL 1.2 PARSECS OF THE GALAXY

We have obtained spectra of the 2.3 μm CO band head in integrated starlight at several positions in the central 1.2 pc of the Galaxy. We find that the strength of the absorption feature declines with projected distance for projected distances less than 15″ (0.6 pc). We confirm that there is an increase in the velocity dispersion σ of the faint stars toward the Galactic center. However, within a 15″ radius of IRS 16, we find that there is no dependence of σ on projected distance. We find that 〈σ〉 = 125 km s-1 and that systematic rotation of the stellar cluster is negligible in the central region. The observations are consistent with there being no CO absorption feature in the diffuse starlight within a true radius of 0.6 pc, with the observed CO feature at projected distances less than 0.6 pc arising in material at larger true radii along the line of sight. We discuss several possible explanations for the lack of CO absorption in the central 1.2 pc, including destruction of the atmospheres of late-type stars by stellar collisions, dissociation of the CO molecule in the atmospheres of the late-type stars by a central luminosity source, and a core radius for the old stellar population of 0.6 pc combined with an additional cluster of sources without CO absorption in the central parsec. Given the new information on the dependence of the strength of the CO band head on projected distance, we have reanalyzed the kinematic data presented in a previous paper (McGinn et al. 1989) to derive the mass distribution for radii between ∼4 and ∼0.6 pc. We confirm the conclusions of that paper that there is evidence for an increase in the mass-to-2 μm radiation ratio M/FK toward the center of the Galaxy. If there is no change in M/FK of the stellar cluster, then an unseen mass of (5.5 ± 1.5) × 106 M⊙ must be concentrated within a 0.6 pc radius of the Galactic center in order to explain the observed kinematics.