In a galaxy seven billion light-years from Earth, at a time when our Sun was just being born, several thousand stars died in massive explosions that blasted giant "superbubbles" into the surrounding clouds of gas and dust. The discovery of this extraordinary event is being presented today at the American Astronomical Society Meeting in San Diego, California, by undergraduate Nicholas A. Bond in collaboration with astronomers Christopher W. Churchill and Jane C. Charlton of the Pennsylvania State University (Penn State) and Steven S. Vogt of the University of California at Santa Cruz.
"A superbubble is a huge, roughly spherical region ten thousand trillion miles across, where thousands of exploding stars have literally blown a hole in the gaseous medium between the stars," says Bond, an astronomy major in his junior year at Penn State, who says his team's research reveals that as many as six of these superbubbles formed at nearly the same time in the galaxy. Charlton, an associate professor of astronomy and astrophysics, adds, "The energy released by the dying stars in those moments that created these gigantic holes is equivalent to the output of 10,000 suns over their entire 10-billion-year lifetimes."
"In a Hubble Space Telescope image we can see the shape and size of the young galaxy from the light of its shining stars; however, we can't actually see the swiss-cheese structure of these superbubbles because the stars whose explosive deaths created it are no longer lighting it up," said Churchill, a research associate in the Department of Astronomy and Astrophysics at Penn State. The clue that lead to the detection of the invisible holes was the astronomers' discovery of the superbubbles' fingerprints--a unique pattern of "absorption lines"--in the spectrum of a quasar whose thin beam of light passes through the galaxy before it reaches Earth.
"We use quasars, which are more luminous than a trillion suns and lie at the outer edges of the universe, as deep-space flashlights," Churchill explains. The absorption-line pattern in the quasar's spectrum revealed the signature "shadow" of the superbubbles along the narrow channel illuminated through the galaxy by the quasar's light. "The striking pattern from this galaxy is several absorption-line pairs, each having a velocity separation of 70,000 miles per hour, which is a gauge of the speed at which the opposite sides of the superbubbles are flying apart," Churchill says.
"We investigated the alternative possibility that two or three galaxies in a small group were giving rise to the absorption-line pairs, but these scenarios could not satisfactorily explain the observations," explains primary investigator Bond. "Superbubbles are the most plausible mechanism through which such evenly split absorption-line pairs could have been created. Because superbubbles are known to have relatively short lifespans, the fact that we see several of them at one time indicates that they all formed at very close to the same time," Bond adds.
Astronomers use radio waves to directly observe emissions from superbubbles in our Milky Way galaxy and in nearby galaxies, but this technique is not adequate for detecting superbubbles in very distant galaxies. The Penn State team says their discovery with quasar light provides the best evidence yet for the existence of superbubbles in a galaxy so distant from Earth. The quasar's light, which took seven billion years to reach Earth after it passed through the superbubble regions, provides a snapshot of how the galaxy looked seven billion years ago, when it was undergoing multiple throes of mass stellar death.
Bond has been doing research with Churchill and Charlton since his freshman year at Penn State and is funded by the National Science Foundation Research Experience for Undergraduates Program. "It has been incredible to work on such cutting-edge research," said Bond. "In classes, I learn about astronomical methods and at work I apply those very same methods to my research." Charlton tells students that the shadows in quasar spectra are "codes" that once broken can reveal the evolution of structure in galaxies like our own. "By breaking the code for each galaxy one at a time," says Charlton, "we hope to some day build a movie-like time line of the universe for instant play back."
The quasar, named MC 1331+170, is located in the constellation of Coma Berenices, in the northern sky near the star Arcturus in Bootes, as is the galaxy in which the superbubbles were discovered. Churchill and Vogt observed this quasar with the High Resolution Spectrograph (HIRES) on the 10-meter W. M. Keck Telescope at the summit of Mauna Kea in Hawaii. Vogt, a professor of astronomy, designed and built HIRES, an instrument that has revolutionized astronomical spectroscopy. He also uses the power of HIRES to successfully hunt for extra-solar planets.
Generous support for this research is provided by the National Aeronautics and Space Administration through the Long Term Space Astrophysics Program and from the National Science Foundation's Extragalactic Astronomy and Research Experiences for Undergraduates Programs.
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