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Hubble Weighs In on Heavy Element Emergence

Astronomers using the Hubble Space Telescope to probe a neighboring galaxy have made a surprising discovery in its cosmic compost. According to findings presented on Monday at the American Astronomical Society's meeting in Pasadena, Calif., the composition of interstellar dust clouds in the Small Magellanic Cloud (SMC) differs from that of the Milky Way's dust clouds. The discovery could force researchers to rethink current ideas about interstellar dust and the evolution of heavy elements.

Daniel Welty of the University of Chicago and his colleagues employed Hubble's imaging spectrograph to comb through the recyclable remnants of dead stars in the SMC. Previously scientists had assumed that the makeup of interstellar clouds is the same from one cloud to another. But absorption line spectroscopy analysis revealed that, in fact, the relative proportions of some heavy elements differ between clouds in the two galaxies. Specifically, whereas the gas in the Milky Way's interstellar clouds lack iron and silicon (which suggests that those elements occupy the dust instead), the gas in the SMC's interstellar clouds exhibits low iron content and high silicon content¿indicating that the dust there contains little silicon.

"This could be a breakthrough in actually measuring the different ways in which solid particles can form and develop in space, which ultimately will help us understand star formation," team member Donald York observes. If the SMC's dust is indeed composed primarily of iron, the researchers note, this could affect stellar development. Says York, "One needs grains to produce molecular hydrogen to produce cooling to produce stars."

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