Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995adil...dr...03r&link_type=abstract
NCSA Astronomy Digital Image Library
Astronomy and Astrophysics
Astronomy
Scientific paper
The ``mini-cavity'' of the Galactic center H II region Sgr A West approximately 2" southwest of Sgr A* has been imaged in the H 92-alpha line at 8.3 GHz with the VLA. The data have an rms noise of 0.16 mJy per beam and spatial and spectral resolutions of 0.75" x 1.18" (alpha x delta) and about 14 km/s, 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 have been used to determine the distribution of electron temperatures. The kinematics of the ionized gas in the inner parsec of the Galactic center have been determined. A large velocity gradient (> 600 km/s per pc) is observed along the eastern rim and uniformly large velocities are observed along the western rim of the mini-cavity. The velocity field of the mini-cavity is interpreted as gas in a hyperbolic orbit about Sgr A*. The kinematics suggested for the ionized gas agree with the model suggested by Zhao, Goss, & Ho (1995) for OH gas which lies outside the mini-cavity (r > 0.6 pc). The similarity of the kinematics of the ionized and molecular gas suggests that the mini-cavity is the ionized component of a molecular cloud impacting close to the Galactic center. The predicted trajectory of this orbit passes within a distance of 0.13 pc of Sgr A*. 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 masses, which is consistent with the estimated mass of the inner parsec (25") of the Galactic center (about 3.5 x 10^6 solar masses). Using the core radius of the central stellar cluster as estimated by Eckart et al. (1993), the stellar mass within the radius of the mini-cavity (about 5 x 10^5 solar masses) is only 20% of the mass estimated using the ionized gas kinematics; thus stars alone cannot explain the observed mass distribution. The electron temperature lies in the range 4000 and 7000 K throughout the mini-cavity and is comparable to the average value of about 7000 K found for Sgr A West. At the position of the near infrared source IRS 13 (located at the northwestern edge of the mini-cavity), a low line-to-continuum ratio (L/C) is observed (< 1%) in the H 92-alpha line in contrast to the Br-gamma emission, in which IRS 13 is one of the brightest sources in the inner parsec of the Galaxy. It is likely that the strong Br-gamma emission at IRS 13 is of stellar origin; a substantial continuum opacity and pressure broadening are responsible for the small L/C of the H 92-alpha line toward IRS 13.
Goss William Miller
Roberts Daniel A.
Yusef-Zadeh Farah
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