Dynamics of Gas in the Galactic Center

  • The High-density Ionized Gas in the Central Parsec of the Galaxy

Abstract We report a study of the H30alpha line emission at 1.3 mm from the region around Sgr A* made with the Submillimeter Array at a resolution of 2" over a field of 60" (2 pc) and a velocity range of -360 to +345 km/s. This field encompasses most of the Galactic center's "minispiral." With an isothermal homogeneous H II model, we determined the physical conditions of the ionized gas at specific locations in the Northern and Eastern Arms from the H30alpha line data along with Very Large Array data from the H92alpha 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 3-21x104 cm-3 and 5000-13,000 K, respectively. The H30alpha and H92alpha 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 three-dimensional 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 pc south of and behind Sgr A*. Finally, a total Lyman continuum flux of 3 x 1050 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 ~250 O9-type zero-age-main-sequence stars.

Zhao, Jun-Hui; Blundell, Ray; Moran, James M.; Downes, Dennis; Schuster, Karl F.; Marrone, Daniel P., 2010, ApJ, 723, 1097

  • Dynamics of Ionized Gas at the Galactic Center: Very Large Array Observations of the Three-dimensional Velocity Field and Location of the Ionized Streams in Sagittarius A West

Abstract We present new results based on high-resolution observations of Sgr A West at the Galactic center with the Very Large Array (VLA) at 1.3 cm. By combining recent observations with those made at earlier epochs with the VLA at wavelengths of 1.3 and 3.6 cm, we measured proper motions for 71 compact H II components in the central 80" (3 pc, assuming D = 8 kpc). Using VLA archival data for the H92alpha radio recombination line, we also investigated radial velocities in the LSR velocity range from +200 to -415 km s-1. Combining proper motion and radial velocity measurements, we have determined the three-dimensional velocity distribution in Sgr A West. We find that the three ionized streams (Northern Arm, Eastern Arm, and Western Arc) in the central 3 pc can be modeled with three bundles of Keplerian orbits around Sgr A*. Assuming that each of the observed streams of ionized gas follows a single orbit, we determined the five orbital parameters (a, e, Omega, omega, i) for each of them using least-square fitting to the loci of the streams. The degeneracy in the orbital solutions for both the direction of flow and the two mirror images can be further resolved using the information obtained from the velocity measurements. 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. All three are confined within the central 3 pc. Both the Northern and Eastern Arm streams have high eccentricities (e = 0.83 +/- 0.10 and 0.82 +/- 0.05, respectively), while the Western Arc stream is nearly circular, with e = 0.2 +/- 0.15. All three streams orbit around Sgr A* in a counterclockwise sense (viewed from the Earth) and have orbital periods in the range 4-8 x 104 yr. To verify the fit, the distributions of radial and transverse velocity vectors in Sgr A West were also computed using the Keplerian model and they show good agreement with both the proper motion and radial velocity data. In addition, the computed orbits suggest that the Northern and Eastern Arm streams may collide in the "Bar" region (a few arcsec south of Sgr A*) and that most of the orbiting ionized gas in the "Bar" region is located behind Sgr A*. We also report an ionized nebula associated with IRS 8, including a bow shock in radio continuum emission which shows excellent agreement with near infrared observations. From the H92alpha line data, we find evidence of substantial interaction between the IRS 8 nebula and the Northern Arm stream occurring in the bow-shock region. 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 that 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.

Zhao, Jun-Hui; Morris, Mark R.; Goss, W. M.; An Tao, 2009, ApJ, 699, 186

  • VLA OH Observations of High Negative Velocity Gas toward Sagittarius A West: A High-Velocity Cloud Interacting with the Galactic Center

Abstract The high negative velocity cloud in the direction of Sgr A* at -180 km s-1 was discussed by Gusten & Downes (1981) more than a decade ago. Since then, numerous observations have been carried out to determine the properties of this component; however, the nature of this feature was uncertain due to the lack of high angular resolution observations. Recently, we have carried out VLA observations of this feature in the H I and OH lines at VLSR = -180 km s-1 with an angular resolution of several arcseconds. The OH 1667 MHz absorption is the best tracer of the gas in front of the continuum sources in the Galactic center. Both the structure and the kinematics have been determined, and we show that the high negative velocity absorbing gas is well mixed with the ionized gas in the center of the Galaxy. The mass of neutral gas seen in absorption at -180 km s-1 is about 5 x 103 Msun, only a small fraction of the entire cloud observed in CO emission. The observed kinematics are not consistent with ejection; outflow, or a possible association with the expanding molecular ring at V = -135 km s-1. The location and morphology suggest that this high-velocity cloud may be tidally disrupted. Based on a numerical model, we demonstrate that the disrupted high-velocity gas can flow toward the Galactic center and interact with the central gravitational potential, thereby distorting the kinematics of the high-velocity gas projected in front of Sgr A West. This model also suggests that the mini-cavity 3" southeast of Sgr A* could have resulted from the impact of a high negative velocity streamer. Both the morphology and the kinematics calculated in this model are consistent with the H92alpha observations of the central 10" of the Galaxy as published by Roberts, Yusef-Zadeh, & Goss (1995).

Zhao, Jun-Hui; Goss, W. M.; Ho, P. T. P., 1995, ApJ, 450, 122