Photo in the News: Galactic "Ghost Arms" Mystery Solved?

Galaxy with four arms image
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April 11, 2007—Looking like a huge interstellar pinwheel, galaxy M106 has stumped scientists trying to decipher the workings of its spiral arms for the past 45 years.

Now, with help from a quartet of space observatories, researchers from the University of Maryland think they might have solved the mystery of what gives the distant galaxy its unusual shape.

Infrared data from NASA's Spitzer Space Telescope revealed two bright spiral arms—shown in green in the composite image above—stretching out from the galaxy's nucleus (see more Spitzer images). These arms are full of young stars that light up the gas around them.

"But in radio [shown in blue] and x-ray [shown in red] images, two additional spiral arms dominate the picture, appearing as ghostly apparitions between the main arms," team member Andrew Wilson said in a press release.

These so-called anomalous arms are mostly made of gas and don't appear to contain stars. So why do the arms light up?

To find out, scientists analyzed archived data from NASA's Chandra X-ray Observatory and the Hubble Space Telescope along with data from the European Space Agencys XMM-Newton X-ray Observatory.

The answer, the researchers say, is most likely that the ghostly arms are regions of gas that are being violently heated by shock waves.

Radio images had previously identified a pair of particle jets shooting out of the supermassive black hole at the galaxy's center (related: "Supermassive Black Hole at Center of Milky Way, Study Hints" [November 2, 2005]). These jets heat the surrounding gas, creating shock waves that in turn superheat the gas to millions of degrees.

Among other clues, the archived Chandra images seem to confirm the latest theory, showing evidence of gas around the galaxy being "shocked" by the jets. The researchers will report their findings in the May 10 issue of the Astrophysical Journal.

—Victoria Jaggard

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