Delicate Diet of a Supermassive Black Hole


Jack Gallimore, Bucknell University
Stefi A. Baum, Space Telescope Science Institute
Christopher P. O'Dea, Space Telescope Science Institute

VLBA Project Codes BG49 (May 1996) & BG66 (April 1997)
MERLIN Project Code MN/97B-10

NGC 1068
NGC 1068 is a relatively nearby spiral galaxy harboring an active galactic nucleus, which is a compact region in the center of the galaxy that produces luminous ultraviolet radiation, X-rays, and a jet of subatomic particles that emits radio waves. An image of the radio jet of NGC 1068, taken with the MERLIN telescope based at the Jodrell Bank Observatory, is shown in the left panel of the figure. The image is reproduced in false color with redder tones indicating fainter radio emission, and lighter gold tones mark brighter radio emission. The extreme, ionizing radiation and radio jet are produced by a superheated accretion disk that feeds gas into a black hole at the center of the galaxy. Feeding the central black hole requires a delicate balance: if the accretion disk feeds the black hole too rapidly, the resulting burst of radiation can blow away the accretion disk, and the black hole is left to starve.

The right panel of the figure zooms in on the central few light years of NGC 1068, as measured by the VLBA. The rainbow-colored spots mark the locations of molecular gas clouds found by Greenhill and Gwinn (1997). The molecular radio emission, actually luminous maser emission from water molecules, traces cooler gas in the outer regions of the accretion disk. Blue-colored spots are rotating toward the observer at 300 km/sec (670,000 mph), and red-colored spots are rotating away at 300 km/sec. Based on these measurements, we know that the mass of the black hole is roughly 10 million times the mass of our Sun.

The red and gold colored contours trace a new VLBA image of radio emission from million-degree gas blowing off of the molecular accretion disk. It appears that the black hole of NGC 1068 is feeding from the accretion disk sufficiently rapidly that the central, X-ray emitting region heats the surface of the molecular accretion disk, causing it to evaporate into a massive wind traveling at approximately 500 km/sec (1 million mph). This X-ray heated wind can remove roughly one solar mass from the molecular accretion disk every year. If no new gas falls in from the surroundings to support the molecular disk, or unless the black hole reduces its voracious appetite, the molecular accretion disk may completely evaporate in roughly 2000 years, which is relatively short compared to the 100 million year lifetime of a typical accretion disk.


Gallimore, Baum, & O'Dea 1997, Nature, 388, 852

Gallimore, Baum, & O'Dea 2004, ApJ, in press (October issue)