In this composite image of M106, radio data from the Very Large Array appears as blue, X-ray data from Chandra is coded red, and infrared data from the Spitzer Space Telescope appears green. The 'ghost' arms appear as purple and blue emissions.

Credit: X-ray: NASA/CXC/Univ. of Maryland/A.S. Wilson et al. IR: NASA/JPL-Caltech; VLA & NRAO/AUI/NSF

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CANBERRA: Using four orbiting space telescopes, astronomers have finally cracked a 45-year old mystery surrounding the origin of two ghostly arms in a nearby galaxy.

Their observations suggest that galaxy M106's strange appendages are regions of gas violently heated by shock waves.

"The nature of these anomalous arms is a long-standing puzzle in astronomy," said astronomer Yuxuan Yang, from the University of Maryland in College Park, U.S.. "They have been a mystery since they were first discovered in the early 1960s."

Located a distance of 23.5 million light years from Earth, M106 looks like a giant rotating cosmological pinwheel.

Most of the matter in the galaxy sits within a thin disc - something like a CD in shape - with arms of gas, dust and stars spiraling out from a bright bulge of tightly packed stars at the centre. Just like our own Milky Way, M106 is called a spiral galaxy because of this pinwheel shape.

In visible light M106 contains only two prominent arms that spiral outward from the bulge. These arms are mostly made up of young, bright stars.

But in radio and X-ray images, "two additional spiral arms dominate the picture, appearing as ghostly apparitions between the main arms," said Andrew Wilson a member of Yang's research team at the University of Maryland. The pair are authors of a study on the phenomenon to be published next month in the Astrophysical Journal.

Among the tightly-packed stars at the centre of the galaxy, astronomers also believe that M106 harbours a supermassive black hole. This black hole spews out two jets of matter, in opposite directions, at close to the speed of light.

Previously these jets were candidates for the ghostly arms, but closer observation showed the jets and arms did not line up. While the ghost-arms lay within the thin disc of the galaxy, the actual jets shoot out a 30° angle from the disc.

Now, using data from space telescopes, Yang's team has shown that the jets do produce the ghost arms, but indirectly.

The jets heat up everything in their path, forming an expanding cone of ionised and energetic matter. Because the jets lie close to the galaxy disk, this cone runs into the disc. This results in shockwave that heats up the gas in the disc to millions of degrees and then forces it out towards the spiral arms – thus creating the ghostly extra arms.

"Often things are puzzling because we only see part of the picture," commented astronomer Rachel Webster from the University of Melbourne, Australia. "These astronomers have cracked the puzzle by imaging the galaxy at different wavelengths," to build up a total picture, she said.

To put all the pieces together, the team used a quartet of orbital space observatories. In the X-ray region, data was collected from NASA's Chandra X-ray observatory and the European Space Agency's XMM Newton X-ray observatory. In the infrared region, researchers used the largest infrared telescope ever launched into space, NASA's Spitzer Space telescope. To obtain information from the optical, ultraviolet and near-infrared regions they used decade-old data from NASA's now decrepit Hubble Space Telescope.

More information:

Yuxuan Yang's web site