Structural analysis of the Minispiral from high-resolution Br? data

Abstract

Integral field spectroscopy of a roughly 40″ × 40″ region about the Galactic Center was obtained at 2.16 μm (Br ) using BEAR, an imaging Fourier Transform Spectrometer, at a spectral resolution of 21.3 km s–1and a spatial resolution of ≃0.5″. The analysis of the data was focused on the kinematics of the gas flows concentrated in the neighborhood of SgrA* , traditionally called the “Minispiral”. From the decomposition into several velocity components (up to four) of the line profile extracted at each point of the field, velocity features were identified. Nine distinguishable structures are described: the standard Northern Arm, Eastern Arm, Bar, Western Arc, as well as five additional moving patches of gas. From this analysis, the Northern Arm appears not limited, as usually thought, to the bright north-south lane seen on intensity images, but consists instead of a continuous, weakly-emitting, triangular-shaped surface having a bright western rim, and narrowed at its forward apex in the vicinity of SgrA* where a strong velocity gradient is observed. The gravitational field of the central Black Hole can account for both the strong acceleration in this region and the tidal compression of the forward tip of the Northern Arm. Keplerian orbits can be fitted to the velocity field of the bright lane, which can be interpreted as formed by the bending of the western edge of the flowing surface. These results raise questions regarding the formation of the Sgr A West gas structures.