The velocity field of ionized gas near Sgr A$^{\ast}$

Abstract

Several distinct kinematic patterns are superposed in the ionized gas seen nearest Sgr A$^{\ast}$, the compact radio source at the center of the Milky Way. This has led to confusion over the morphology and motions of the gas; the recent dispersion-ring model of Sanders (1998) inadvertantly elides parts of two separate features and reproduces a description of the gas flow which does not actually occur. We demonstrate the gas kinematics via an analysis of the high-resolution H92 α radio recombination line data of Roberts & Goss (1993) and Roberts et al. (1996). The characteristic kinematic pattern of the “Bar” is reproduced by rotation in near-polar orbits seen edge-on at distances of 0.3–0.8 pc from Sgr A$^{\ast}$, with larger radii seen to the east. The mass of ionized material in the Bar is ~$27~{ M}_{\odot}$. The Bar and Eastern Arm together probably represent a single stream of gas falling on to the galactic center, crossing from east to west.