X-Ray Chaos: Violence shows itself in a nearby galaxy

Only 11 million light-years from Earth, Centaurus A is the nearest galaxy powered by a supermassive black hole that emits radio waves. The galaxy also sends out jets of charged particles and appears to have swallowed a smaller galaxy 100 million years ago (SN: 5/16/98, p. 311). Now, astronomers have found further evidence that Centaurus A is a maelstrom of violence.

NASA’s Chandra X-ray Observatory has discovered a pair of giant gas arcs in the galaxy, at temperatures of multimillion degrees, that are part of a 25,000-light-year-wide ring aglow in X rays. The size and orientation of the ring suggest that something stirred up the galaxy about 10 million years ago. Margarita Karovska, Stephen S. Murray, and their colleagues at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., report their observations in the Sept. 20 Astrophysical Journal. Previous views in both visible and infrared light had revealed that star formation in the galaxy also revved up about 10 million years ago.

One explanation for the ring is that it formed in the tumultuous aftermath of the collision in which Centaurus A snared the smaller galaxy. However, because galaxy collisions are extremely messy, the ring’s uniformity argues against that scenario, notes F. Duccio Macchetto of the Space Telescope Science Institute in Baltimore.

Another possibility is that some disruption at the galaxy’s core 10 million years ago, perhaps a sudden increase in the amount of matter sucked in by the black hole, triggered an expanding shock wave, says Murray. When the speeding wave slammed into dense parts of the interstellar medium, its kinetic energy would have been converted into X rays.

Although the observations were taken in 1999, Karovska found the diffuse arcs only recently, after carefully blurring the X-ray image to make such subtle features stand out.