NEON II OBSERVATIONS OF THE GALACTIC CENTER: THE VELOCITY FIELD AND MASS DISTRIBUTION

The velocity field within 2 pc of the galactic center has been investigated by observing the NeII 12.8 (mu)m fine-structure line emission from Sgr A West. The observations show regular variations in velocity along several of the most prominent radio continuum features, indicating the presence of at least two large-scale flows of ionized gas. One of the flows follows a circular orbit at a galacto- centric radius of (DBLTURN) 1.7 pc, and requires 4.7 x 10('6) M(,o) to lie within that radius. The other flow is well fit by an eccentric orbit about a point mass. This orbit approaches within 0.5(, )pc of the center of the galaxy and requires >(, )3.5 x 10('6) M(,o) to lie within that radius. The inferred mass distribution is too centrally(, )peaked to be due to a spherical isothermal stellar cluster, implying that a substantial fraction of the mass located within 1.7 pc of the center is contained in a centrally condensed form, possibly in a massive black hole of mass (DBLTURN) 3-4 x 10('6) M(,o). The best fit orbits occur under the assumption that the point mass is near the 2 (mu)m source IRS 16 NE.

The flow at r (DBLTURN) 1.7 pc is most probably located on the inner edge of the ring of dust and neutral gas which orbits about the center at larger radii. Consequently, the source or sources of the ionization must be centrally located. Shielding of various portions of the ring from the central ionizing flux may play a role in determining the distribution of the ionized gas in Sgr A West. If so, the source of the ionization can be localized to within (DBLTURN) 0.5 pc of the center. A single source of ionization with T(,eff) (DBLTURN) 35,000 K and L(,bol) (DBLTURN) 10('7) L(,o) is consistent with the ionization state of the gas in the central 3 pc, the bolometric luminosity of the central few parsecs and also the near infrared flux from IRS 16 center. Thus, a single object may be the primary source of energy for the central few parsecs of our galaxy.