In some of the richest, most tumultuous parts of spiral galaxies — the dusty gas clouds veiling embryonic stars — molecules lurk that could serve as life’s Legos. These aromatic hydrocarbons are carbon-based compounds found nearly anywhere combustion occurs — from stellar nurseries to terrestrial barbeque pits. They appear abundantly in the interstellar dust of the Milky Way and nearby galaxies.
But Karl Gordon (Space Telescope Science Institute) and his colleagues claim that their Spitzer Space Telescope observations reveal something strange about the polycyclic aromatic hydrocarbons (PAHs) in the outer rim of M101, a big spiral galaxy just off the Big Dipper. Here, there aren’t any.
The dearth appears as the red patches in the infrared image here. Both the M101 study and observations of starburst galaxies led by one of the paper’s co-authors, Charles Engelbracht (University of Arizona), find a correlation between the PAH decline and an increase in the ionization of hot hydrogen regions. The astronomers think that the dead zones exist because harsh radiation from hot young stars destroys organic molecules. Radiation from stars in galaxies’ outer regions should be more damaging to organics, because these stars have a lower heavy-element content than stars closer to the galactic core, and high-energy radiation can more easily pass through their atmospheres. This radiation would also ionize the hydrogen regions.
At the same time, PAHs have a major role in star formation. Due to the way they absorb and re-emit radiation, PAHs help lower the temperature in a molecular cloud. Low temperatures are important because a cloud must be cold enough to clump up before stars can be born inside it: the clumps collapse under their own gravity to form the stars. If gas is hot, it exerts too much pressure to clump well. Since the PAH dust did not exist in the early universe, observing star formation in places like M101’s outer rim today allows astronomers to study how the first stars might have coalesced soon after the cosmos’s dawn.