'Ghost' Spirals Born From Black Hole

April 19, 2007 — Mysterious invisible "ghost spirals" observed in a nearby galaxy appear to be gigantic super-hot shockwaves, say astrophysicists.

The ghostly spiral arms of galaxy M106 were first detected in radio waves, contain no stars, and do not match up with the curved starry arms of the galaxy as seen by telescopes in visible light.

But new observations of M106's "anomalous arms" confirm they are the result of two gigantic jets blasting at a strange angle from the galaxy's central supermassive black hole.

"All you see is gas that is heated by shock waves," said astrophysicist Andrew Wilson of University of Maryland, referring to what the ghost arms are made of.

Because M106 is a relatively nearby galaxy — just 23.5 million light years away — new technology has allowed astronomers to continually improve their measurements of what's happening around M106's black hole and in its arms.

A few years ago, astronomers using the super-high resolution array of radio telescopes called the Very Long Baseline Array (VLBA) were finally able to measure the angle of those black hole jets roaring out of the core of M106, said Lincoln Greenhill of the Harvard-Smithsonian Center for Astrophysics. His team made the VLBA observations.

"The innovation now is recognizing the orientation of jets in three dimensional space," Greenhill told Discovery News. "This ends up blasting away the assumption that galaxies are flat."

The black hole doesn't know the orientation of the galaxy it's inside of, Greenhill explained. That's why the black hole's jets can project at an angle about 30 degrees out of kilter with the rest of the galactic disk. Most theories would expect the jets to project perpendicular to the disk, like a light shining up and down out of the center of a flying saucer.

"It's a major misalignment," Greenhill observed.

Now Wilson and a team led by colleague Yuxuan Yang have taken the matter even further to find out the three-dimensional positions of the entire arms.

To do that they used observations from NASA's orbiting Chandra X-Ray Telescope, Spitzer Space Telescope, the Hubble Telescope and the European Space Agency's XMM-Newton X-Ray Observatory, to confirm that one arm is definitely above the galaxy and the opposite arm below it, matching the orientation of the jets.

"The jets are driving out, at a funny angle, shocks into the galactic disk," Wilson told Discovery News. "So the anomalous arms are the regions that are heated by the jets." Wilson, Yang and their colleagues published the discovery in the May 10 issue of Astrophysical Journal.

"In some sense they have proposed something that is very beautiful," said Greenhill of the elegant simplicity of what Wilson and his team propose is creating the ghost arms. What's more, he said, M106 could be tutoring astrophysicists on the sort of things that cause other odd features in other galaxies.