KINEMATICS OF THE IONIZED GAS IN SAGITTARIUS A WEST: MASS ESTIMATES OF THE INNER 0.13 PARSECS OF THE GALAXY

摘要

The "minicavity" of the Galactic center H II region Sgr A West, ～3" southwest of Sgr A~*, has been imaged in the H92α line at 8.3 GHz with the VLA. The data have an rms noise of 0.16 mJy beam~(-1) and spatial and spectral resolutions of 0.75″ x 1~(-1)8 (α x δ) and ～14 km s~(-1), respectively. These are the highest spatial and velocity resolution observations of the ionized gas at the Galactic center to date, and they provide an estimate of the mass distribution close to the dynamical center of the Galaxy. In addition, the observed line-to-continuum ratios (L/Cs) have been used to determine the distribution of electron temperatures. The kinematics of the ionized gas in the inner parsec of the Galactic center has been determined. A large velocity gradient ( >600 km s~(-1) pc~(-1)) is observed along the eastern rim, and uniformly large velocities are observed along the western rim, of the minicavity. The velocity field of the minicavity is interpreted as that of gas in a hyperbolic orbit about Sgr A~*. The kinematics suggested for the ionized gas agrees with the model suggested by Zhao, Goss, & Ho for OH gas that lies outside the minicavity (r > 0.6 pc). The similarity of the kinematics of the ionized and molecular gases suggests that the mini-cavity is the ionized component of a molecular cloud, the orbit of which comes close to the Galactic center. The predicted trajectory of this orbit passes within 0.13 pc of Sgr A~*, assuming a distance to the Galactic center of 8.5 kpc. Based on this model, the gas is orbiting a point at the position of Sgr A~* with a mass of (3.0 ± 0.5) x 10~6 solar mass, which is consistent with the estimated mass of the inner parsec (25″) of the Galactic center (～3.5 x 10~6 solar mass). Using the core radius of the central stellar cluster as estimated by Eckart et al., the mass from luminous stars within the radius of the minicavity (～5 x 10~5solar mass) is only 20% of the mass estimated using the ionized-gas kinematics. The electron temperature lies in the, range 4000-7000 K throughout the minicavity and is comparable to the average value of ～7000 K found for Sgr A West. At the position of the near-infrared source IRS 13 (located at the northwestern edge of the minicavity), a low L/C is observed (< 1%) in the H92α line in contrast to the Brγ emission, in which IRS 13 is one of the brightest sources in the inner parsec of the Galaxy. It is likely that the strong Brγ emission at IRS 13 is of stellar origin: a substantial continuum opacity and pressure broadening are likely responsible for the small L/C of the H92α line toward IRS 13.