Dynamics of Ionized Gas at the Galactic Center: VLA Observations of the
  3D Velocity Field and Location of the Ionized Streams in Sagittarius A West
  We present new results based on high-resolution observations of Sgr A West at
the Galactic center with the VLA at 1.3 cm. We measured proper motions for 71
compact HII components. We also investigated radial velocities in the LSR
velocity using the H92a line data. Combining proper motion and radial velocity
measurements, we have determined the 3D velocity distribution in Sgr A West. We
find that the three ionized streams (Northern Arm, Eastern Arm, and Western
Arc) can be modeled with three bundles of Keplerian orbits around Sgr A*. We
determined the five orbital parameters for each of them using LSQ fitting to
the locii of the streams. Our results confirm earlier results on the streams in
the Western Arc and the Northern Arm to be in Keplerian orbits, suggesting that
the stream in the Eastern Arm is also consistent with an elliptical orbit. Both
the Northern and Eastern Arm streams have high eccentricities, while the
Western Arc stream is nearly circular. All three streams orbit around Sgr A* in
a counterclockwise sense (viewed from the Earth). We also report an ionized
nebula associated with IRS 8, including a bow shock in radio continuum emission
which shows excellent agreement with near IR observations. From the H92a line
data, we find evidence for interaction between the IRS 8 nebula and the
Northern Arm stream. Other new morphological features revealed in our
high-resolution image include: 1) a helical structure in the Northern Arm,
suggesting that MHD plays an important role in the motion of the ionized gas,
in addition to the dynamics determined by the central gravitational field and
2) a linear feature in the IRS 16 region, suggesting the compressed edge of the
Northern Arm may result from the collective winds and radiation pressure from
the high mass stars in the IRS16 cluster.
