'No organics' zone circles Pinwheel galaxy
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 22, 2008

The Pinwheel galaxy, which is dominated by tangled spiral arms, has been observed through Spitzer’s infrared eyes, revealing a zone in which organic molecules suddenly disappear.

In this image, infrared light with a wavelength of 3.6 microns is coloured blue; 8-micron light is green; and 24-micron light is red. All three of Spitzer instruments were used in the study: the infrared array camera, the multiband imaging photometer and the infrared spectrograph. Image: NASA/JPL-Caltech/STScI.

Officially named Messier 101, and located 27 million light years away in the constellation Ursa Major, the Pinwheel galaxy is seen by the Spitzer Space Telescope in a new ‘paint-by-numbers’ view in which the swirling centre is coloured in blue and a unique coral red ring circles its outer regions. According to a paper published in the July 20 issue of the Astrophysical Journal, the red colour highlights a zone where organic molecules, known as polycyclic aromatic hydrocarbons (PAHs), suddenly disappear. PAHs are dusty, carbon-containing molecules common to star nurseries throughout the Galaxy, and are believed to form some of the building blocks for life.

"If you were going to look for life in Messier 101, you would not want to look at its edges," says Karl Gordon of the Space Telescope Science Institute in Baltimore. "The organics can't survive in these regions, most likely because of high amounts of harsh radiation." 

The Pinwheel galaxy has an extremely high concentration of metals in its centre, as a result of the central stars being squeezed more tightly there, which declines rapidly towards the edges. The astronomers studied this gradient and found that, like the metals, the PAHs also decrease in concentration toward the outer portion of the galaxy. But, unlike the metals, these organic molecules quickly drop off and are no longer detected at the very outer rim, presumably because they are no longer protected by the metals which act to dampen the savage stellar radiation.

"There's a threshold at the rim of this galaxy, where the organic material is getting destroyed," says Gordon.

Because the early Universe did not contain a lot of metals or PAHs, the findings provide a better understanding of the conditions under which the very first stars and galaxies arose.

Under normal circumstances, the PAHs help cool down star-forming clouds, allowing them to collapse into stars. In regions like the rim of the Pinwheel, stars form without the organic dust. Astronomers don't know precisely how this works, so the outskirts of the Pinwheel galaxy provides them with a laboratory for examining the process relatively close up, and gives them an insight into what the environment might look like in a distant galaxy.

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