Astronomers find clues to heavy elements in universe

This image of a portion of the Small Magellanic Cloud was taken by the Hubble Space Telescope. Welty and his colleagues used the HST imaging spectrograph to probe the space between the stars of the Small Magellanic Cloud.

University astronomers have peered into the cosmic recycling bin of a neighboring galaxy and found what may force astronomers to reconsider some of their ideas about interstellar dust and the build-up of heavy elements in the universe, said Daniel Welty, a Senior Research Associate in Astronomy & Astrophysics. Welty presented the findings at the 198th meeting of the American Astronomical Society held last month.

The study may provide new insights into the evolution of galaxies and the creation of heavy elements over a period of time that stretches back to within 1 billion years of the big bang. Only the lightest elements, such as hydrogen and helium, were created during the big bang. The heavy elements that were essential to the formation of Earth and other rocky planets, including silicon, iron, nickel and zinc, were created much later, in exploding stars.

Silicon, for example, is manufactured in explosions of massive stars that have lifetimes of millions of years. Iron is produced primarily in explosions of lighter, sun-like stars that reach the end of their lives after a billion years or more.

Before the Chicago study, it appeared that more silicon than iron was produced early in the lifetimes of young, distant galaxies, as is thought to have been the case for the Milky Way. Now, however, it seems possible that silicon and iron may have been produced in comparable amounts in those distant galaxies.

“It’s a piece of the puzzle of trying to understand the build-up of the heavy elementsóby establishing what the behavior is locally in interstellar matter and then using that information to interpret more distant galaxies that are much earlier in their evolutionary state,” said Welty. The study’s co-authors are Lewis Hobbs, Professor in Astronomy & Astrophysics; Donald York, the Horace Horton Professor in Astronomy & Astrophysics; James Lauroesch, Northwestern University; and Chris Blades, the Space Telescope Science Institute.

Using the Hubble Space Telescope’s imaging spectrograph, Welty and his colleagues probed the seemingly empty space between the stars of the Small Magellanic Cloud, a small galaxy on the far outskirts of the Milky Way, for hints of the cosmic recycling process. The wispy ashes of dead stars that collect in this space as gas and dust provide both a record of previous generations of stars and the raw material for the creation of new stars.

A technique called absorption line spectroscopy enables scientists to determine what elements exist and in what quantities they exist in interstellar gas clouds. But the composition of the gas in the clouds tells only part of the story. Some elements appear to be largely missing from the gas and to be locked in solid dust grains instead.

Because it is difficult to directly measure the composition of the dust, astronomers generally have assumed that the overall ratios of the elements and the general constituents of the mixture of dust grains have largely been the same from one interstellar cloud to another. Based on previous observations made of interstellar clouds in the Milky Way, they had no reason to assume otherwise. But now, Welty and his colleagues have observed that the relative proportions of some elements in the gas and dust in the Small Magellanic Cloud are different from those typically found in Milky Way clouds.

Welty’s team took measurements along a line of sight between Earth and the star Sk 155 in the Small Magellanic Cloud, approximately 200,000 light years away. They use their observations of the Small Magellanic Cloud to help understand the evolution of heavy elements in younger, more distant galaxies, which are more difficult to observe. In interstellar clouds in the Milky Way, both iron and silicon are generally largely absent from the gas, and thus, are thought to be major constituents of the dust.

Toward the star Sk 155, however, the team discovered clouds with very low iron abundance but surprisingly high silicon abundance in the gas. This seems to imply that little, if any, silicon is present in the dust there. If the dust in the more distant galaxies also is deficient in silicon, then the total (gas plus dust) silicon abundance would be smaller than previously thought.

“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,” said York.