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

T. Paumard1, J.P. Maillard1, M. Morris2

1 Institut d'Astrophysique de Paris (CNRS), 98b Blvd Arago, 75014 Paris, France
2 University of California, Los Angeles, Div. of Astronomy, Dept. of Physics and Astronomy, Los Angeles, CA 90095-1562, USA

E-mail contact: paumard@iap.fr

Integral field spectroscopy of the inner region of the Galactic Center, over a field of roughly 40'' x 40'' was obtained at 2.16um (Br gamma) using BEAR, an imaging Fourier Transform Spectrometer, at a spectral resolution of 21.3km.s-1, and 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, with in addition five 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 only to the geometry and 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 lead to a new formation scenario for the gas structures comprising Sgr A West, one in which large-scale gas flows coexist with transient, moving gas patches in the gravitational field of SgrA. The question of the origin of the ionized gas in Sgr A West is addressed, and the lifetime of these features is discussed.