Gas Dynamics

High resolution observations of the Galactic Center at radio wavelengths have revealed a complex structure of ionized gas called the 'mini-spiral'. The mini-spiral is broken down into individual components including the Northern Arm, Western Arc and Eastern Arm (see below). The mini-spiral is surrounded by a thick ring of molecular material called the Circumnuclear Disk (CND) which is from 2.5 to 4.8 parsecs (50 to 95 arcseconds) in size with an inferred inclination of 60 degrees assuming a circular structure (Gusten et al 1987). It is primarily seen in radio molecular emission from carbon monoxide (CO) or hydrogen cyanide (HCN) molecules.

VLA image of the GC at 6cm

The motion of the gas comprising the mini-spiral is determined using the Doppler shift of emission lines including the NeII and Bracket gamma hydrogen lines. These studies show that the gas along the Nothern Arm is falling in toward or orbiting around Sgr A*. There are many ideas about the origin of the mini-spiral. Some believe that the it is created by the same type of gravitational perturbation which forms the pretty pattern observed in spiral galaxies. One theory explaining the motion of the gas in the mini-spiral suggests the gas is in a Keplerian orbit around the supermassive black hole. The other interpretation is that the gas is not bound to the galactic center and therefore is on a hyperbolic orbit. Another scenario for the formation of the mini-spiral involves the collision of gas cloudlets within the CND which causes the clouds the lose angular momentum and fall in towards Sgr A*. Indeed the CND appears to have a clumpy structure and the ionized filaments along the mini-spiral are coincident with the inner edge of the CND and appear to terminate in the CND which, in turn, has an impression at the termination point. The CND is not believed to be in dynamical equilibrium meaning that if cloud collisions within the CND are responsible for creating the mini-spiral then material must be provided to replenish the CND. Some of the material could be coming from a nearby Giant Molecular Cloud (GMC) called the "20 km/s cloud".