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 parsec of the Galactic center at three epochs spanning 1986 to 1995. The Very Large Array A configuration was used at X = 2 cm (resolution of ″1 × 0″2). We detect proper motions of a number of features in the northern and eastern arms of Sgr A West including the ionized gas associated with 1RS 13 with V(R.A.) = 113 ±10 and V(decl.) = 150 ±15 km s^-1, 1RS 2 with V(R.A.) = 122 ±11 and V(decl.) = 24 ±34 km s^-1, and the northern arm with V(R.A.) = 126 ±30 and V(decl.) = -207 ±58 km s^-1. We also report the detection of features having transverse velocities greater than 1000 km s~', including a head-tail radio structure, the "bullet, " ≈4″ northwest of Sgr A* with V(R.A.) = 722 ±156, V(decl.) = 832 ±203 km s^-1, exceeding the escape velocity at the Galactic center. The proper-motion measurements, when combined with previous H92α radio recombination line data, suggest an unambiguous direction of the flow of ionized gas orbiting the Galactic center. The measured velocity distribution suggests that the ionized gas in the northern arm is not bound to the Galactic center assuming 2.5 ′ 10⁶ M_⊙ of dark matter residing at the Galactic center. This implies that the stellar and ionized gas systems are not dynamically coupled, thus supporting a picture in which the gas features in the northern arm and its extensions are the result of an energetic phenomenon that has externally driven a cloud of gas into the Galactic center.