The High-Density Ionized Gas in the Central Parsec of the Galaxy

We report a study of the H30$\alpha$ line emission at 1.3 mm from the region around Sgr A* made with the Submillimeter Array at a resolution of 2\arcsec
over a field of 60\arcsec\ (2 parsec) and a velocity range of -360 to +345 \kms. This field encompasses most of the Galactic center’s “minispiral”. With an isothermal homogeneous HII model, we determined the physical conditions of the ionized gas at specific locations in the Northern and Eastern Arms from the H30$\alpha$ line data along with Very Large Array data from the H92$\alpha$ line at 3.6 cm and from the radio continuum emission at 1.3 cm. The typical electron density and kinetic temperature in the minispiral arms are 3-21$\times {10}^4$ cm${}^{-3}$ and 5,000-13,000 K, respectively. The H30$\alpha$ and H92$\alpha$ line profiles are broadened due to the large velocity shear within and along the beam produced by dynamical motions in the strong gravitational field near Sgr A*. We constructed a 3D model of the minispiral using the orbital parameters derived under the assumptions that the gas flows are in Keplerian motion. The gas in the Eastern Arm appears to collide with the Northern Arm flow in the “Bar” region, which is located 0.1-0.2 parsec south of and behind Sgr A*. Finally, a total Lyman continuum flux of $3\times {10}^{50}$ photons s${}^{-1}$ is inferred from the assumption that the gas is photoionized and the ionizing photons for the high-density gas in the minispiral arms are from external sources, which is equivalent to $\sim 250$ O9-type zero-age-main-sequence stars.