* Astronomy

The Galaxy Evolution Explorer spacecraft has taken a multi-wavelength, neon-coloured image of the 15 magnitude Cartwheel galaxy that lies 420 million light years away in the constellation Sculptor .

The false-colour composite image shows the Cartwheel galaxy in ultraviolet light (blue); the Hubble Space Telescope in visible light (green); the Spitzer Space Telescope in infrared (red); and the Chandra X-ray Observatory (purple).

"The dramatic plunge has left the Cartwheel galaxy with a crisp, bright ring around a zone of relative calm. Usually a galaxy is brighter toward the centre, but the ultraviolet view indicates the collision actually smoothed out the interior of the galaxy, concentrating older stars and dust into the inner regions. It's like the calm after the storm of star formation" - Phil Appleton, astronomer at the California Institute of Technology, Pasadena, California, US.

The outer ring, which is bigger than the entire Milky Way galaxy, appears blue and violet in the image.
Approximately 100 million years ago, a smaller galaxy plunged through the heart of Cartwheel galaxy, creating ripples of brief star formation. Recently-observed features include concentric rings rippling out at a rate of more than 300,000 km/hour, from the impact area in a series of star formation waves, ending in the outermost ring.


Expand (1.36mb, 1500 x 1500)
Position(2000): RA = 0h 37m 41.11s Dec = -33d 42m 58.8s
Image Credit: NASA, JPL, Caltech, R. Hurt (SSC). Text Credit: NASA, JPL, Caltech, Jane Platt (JPL).
The blue outer ring is so powerful in the GALEX observations that it indicates the Cartwheel is one of the most powerful UV-emitting galaxies in the nearby universe. The blue colour reveals to astronomers that associations of stars 5 to 20 times as massive as our sun are forming in this region. The clumps of pink along the outer blue ring are regions where both X-rays and UV radiation are superimposed in the image. These X-ray point sources are very likely collections of binary star systems containing a blackhole (called Massive X-ray Binary Systems). The X-ray sources seem to cluster around optical/UV bright supermassive star clusters.
The yellow-orange inner ring and nucleus at the centre of the galaxy result from the combination of visible and infrared light, which is stronger towards the centre. This region of the galaxy represents the second ripple, or ring wave, created in the collision, but has much less star formation activity than the first (outer) ring wave. The wisps of red spread throughout the interior of the galaxy are organic molecules that have been illuminated by nearby low-level star formation. Meanwhile, the tints of green are less massive, older visible light stars.

"It's like dropping a stone into a pond, only in this case, the pond is the galaxy, and the wave is the compression of gas. Each wave represents a burst of star formation, with the youngest stars found in the outer ring" - Phil Appleton.

Previously, scientists believed the ring marked the outermost edge of the galaxy, but the latest Galaxy Evolution Explorer observations detect a faint disk, not visible in this image, that extends to twice the diameter of the ring. This means the Cartwheel is a monstrous 2.5 times the size of the Milky Way.

Most galaxies have only one or two bright X-ray sources, usually associated with gas falling onto a black hole from a companion star. The Cartwheel has a dozen, because black holes thrive in areas where massive stars are forming and dying fast.
The Cartwheel galaxy is one of the brightest ultraviolet energy sources in the local universe (z = 0.030187). In some visible-light images, it appears to have spokes. Appleton presented his finding at the 207th meeting of the American Astronomical Society in Washington.

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-- Edited by Blobrana at 15:51, 2006-01-12