NCSA Astronomy Digital Image Library
Project Abstract and Description

Multiconfiguration VLA H92alpha Observations of Sgr A West at 1 arcsec Resolution

D.A. Roberts & W.M. Goss

Contact: Doug Roberts
Email: droberts@astro.uiuc.edu

Number of images in project: 5

References: 1993ApJS...86..133R

Abstract

H 92-alpha observations of Sagittarius A (Sgr A) West have been carried out with almost an order of magnitude greater resolution and sensitivity than the previous 2 cm H 76-alpha line observations. These observations have a factor of two greater angular and velocity resolution than recent [NeII] observations. Globally, kinematic features of the western arc, the northern arm and the extended bar have been identified. The western arc has been modeled as half of a ring in circular rotation about a point near the non-thermal point source, Sgr A*, with an inner radius of 25" (1 pc). The modeled ring has a position angle of 22 degrees and an inclination of 56 degrees with a rotational velocity of 105 km/s. The mass interior to the modeled ring at a radius of 1 pc is approximately 3.5 x 10^6 solar masses. The global kinematics have been determined and suggest that the three major kinematic features are all unrelated. This interpretation of the global structure consisting of distinct features differs from the interpretation suggested by the recent [NeII] observations in which all of the gas in Sgr A West is part of a single Keplerian disk.

The H 92-alpha line emission has been compared to the Br-gamma emission in the HII region. This comparison shows the striking result that the Br-gamma emission is absorbed at the outer edge of the western arc. The extinction is strongly correlated with presence of HCN emission from the CND. Given the previous result that the western part of the CND is inclined out of the plane of the sky along with the suggestion that Br-gamma emission from the western arc is absorbed by the CND, it is likely that the western arc has the same orientation as the CND, and is in fact related to the CND.

The properties of the ionized gas near Sgr A* have also been determined. The earlier suggestion of high electron temperatures has been explained as the superposition of a weak spectral feature at v(LSR) = 0 km/s associated with the bar and a strong high negative velocity feature at v(LSR) = -250 km/s associated with the northern arm. Previous RRL experiments did not have sufficient velocity coverage to observe the high negative velocity line and thus overestimated the electron temperature. Taking into account both spectral features, the bar as well as the entire HII region have essentially uniform electron temperatures of approximately 7000 K. The bright continuum emission in the region surrounding the mini-cavity in the bar south of Sgr A* is identified with the northern arm. The southern part of the northern arm, rather than the bar, may be in close physical proximity to Sgr A*. The bar may be a relatively distant feature compared to the northern arm and Sgr A*.

Stimulated emission from Sgr A* is interpreted as occurring in a cloud of ionized gas near the interior edge of the CND. It is possible that this cloud may be falling into the HII region from the CND. The ratio of singly ionized helium to singly ionized hydrogen has been possibly determined at a single position in the northern arm. The He 92-alpha line has been detected at the 2.5-sigma level and a value for Y+ of approximately 5% has been derived. The line-of-sight component of the magnetic field in the northern arm has been investigated and a 3-sigma upper limit of 8 mG has been derived.

NCSA ADIL Home Page
NCSA Astronomy Digital Image Library / adil@ncsa.uiuc.edu