“Organic Zones and Dead Zones” of the Pinwheel Galaxy


The Pinwheel galaxy, 27 million light-years away in the constellation Ursa Major, otherwise known as Messier 101, is gilded by bright reddish edges in this new infrared image from NASA's Spitzer Space Telescope. M101 is nearly twice the size of the Milky Way with huge and extremely bright H II regions, which usually accompany enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force where stars form.

Research from Spitzer has revealed that this outer red zone lacks organic molecules present in the rest of the galaxy. The red and blue spots outside of the spiral galaxy are either foreground stars or more distant galaxies.

The organics, called polycyclic aromatic hydrocarbons, are dusty, carbon-containing molecules that help in the formation of stars. On Earth, they are found anywhere combustion reactions take place, such as barbeque pits and exhaust pipes. Scientists also believe this interstellar dust has the potential to be converted into the stuff of life.

Spitzer found that the polycyclic aromatic hydrocarbons decrease in concentration toward the outer portion of the Pinwheel galaxy, then quickly drop off and are no longer detected at its very outer rim. According to astronomers, there's a "dead zone" threshold at the rim where the organic material is being destroyed by harsh radiation from stars. Radiation is more damaging at the far reaches of a galaxy because the stars there have less heavy metals, and metals dampen the radiation.

The findings help researchers understand how stars can form in these extreme environments, where polycyclic aromatic hydrocarbons are lacking. Under normal circumstances, the hydrocarbons help cool down star-forming clouds, allowing them to collapse into stars. In regions like the rim of the Pinwheel as well as the very early universe stars form without the organic dust. Astronomers don't know precisely how this works, so the rim of the Pinwheel provides them with a laboratory for examining the process close up.

The Daily Galaxy Via NASA/Spitzer


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