What is happening to the gas in the Galactic Center?


VLA image of the Galactic Center at 6 cm.

As we saw before, we can trace ionized gas at radio wavelengths. High-resolution observations of the Galactic Center at radio wavelengths have revealed a structure called the "mini-spiral." The mini-spiral has several different parts--the Northern Arm, the Wester Arc, and the Eastern Arm.


1.3 cm image of the mini-spiral. Source: Zhao et al. 2009

The motion of the gas in the mini-spiral can be determined using the Doppler shift of emission lines, particularly Ne II and Bracket-gamma hydrogen lines. These studies show that the gas along the Northern Arm is falling in toward, or obriting around, the central black hole. Current evidence suggests that the three arms are orbiting the supermassive black hole according to Kepler's laws, and that the gas in the Northern Arm is actually interacting with a nearby source. The Eastern Arm is actually colliding with the Northern arm flow, and that the gas is photoionized--electrons are being stripped off (Zhao et al. 2009).


Molecular gas (HCN) in the circumnuclear disk. Credit: Leo Blitz, University of Maryland

In addition to having different arcs and arms, the mini-spiral is surrounded by a thick ring of molecular material called the Circumnuclear Disk (CND). The origin of this disk is unclear, but it is a ring, or a torus, with an inner radius of about 1 parsec and an outer radius of several parsecs (Note: 1 parsec is 1.917 X 10^13 miles!). This disk's rotation follows Kepler's laws, and it is heated by both central region of old stars and the ultraviolet (short wavelength) flux of bright young stars that are near the black hole.