Proper Motions of Ionized Gas at the Galactic Center: Evidence for Unbound Orbiting Gas

We present radio continuum observations of the spiral-shaped ionized feature(Sgr A West) within the inner pc of the Galactic center at three epochsspanning 1986 to 1995. The VLA A-configuration was used at $\lambda$2cm(resolution of 0\dasec1$\times$0\dasec2). We detect proper motions of a numberof features in the Northern and Eastern Arms of Sgr A West including theionized gas associated with IRS 13 with V(RA)= 113 \pm 10, V(Dec)=150 \pm15km/s, IRS 2 with V(RA)= 122 \pm 11, V(Dec)=24 \pm 34 km/s and the Norther ArmV(RA)= 126 \pm 30, V(Dec)=--207 \pm 58 km/s. We also report the detection offeatures having transverse velocities > 1000 km/s including a head-tail radiostructure, the ``Bullet'', $\approx4''$ northwest of Sgr A$^*$ with V(RA)= 722\pm 156, V(Dec)=832 \pm 203 km/s, exceeding the escape velocity at the Galacticcenter. The proper motion measurements when combined with previous H92$\alpha$ radiorecombination line data suggest an unambiguous direction of the flow of ionizedgas orbiting the Galactic center. The measured velocity distribution suggeststhat the ionized gas in the Northern Arm is not bound to the Galactic centerassuming a 2.5 million solar mass of dark matter residing at the Galacticcenter. This implies that the stellar and ionized gas systems are notdynamically coupled, thus, supporting a picture in which the gas features inthe Northern Arm and its extensions are the result of an energetic phenomenonthat has externally driven a cloud of gas cloud into the Galactic center.Comment: 11 pages, three figures (one color) and one table. Astrophysical Journal Letters in pres