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SURPRISING BEHAVIORS OF YOUTHFUL GALAXIES CHALLENGE ACCEPTED THEORIES ABOUT THEIR FORMATION
Clouds of primordial gas located near the outer reaches of the Universe appear to be infant galaxies whose behaviors are surprisingly similar to their grown-up counterparts, including our own Milky Way.
According to new studies by astrophysicists at the University of California, San Diego, the fact that these protogalaxies form relatively thick disks that rotate as rapidly as the mature galaxies we see today contradicts accepted theory.
"We're not saying that the cosmology is wrong," said Arthur Wolfe, professor of physics at UCSD and coauthor of the study with graduate student Jason X. Prochaska. "We're saying the part of the standard lore concerning galaxy formation needs to be changed."
Wolfe and Prochaska, both researchers in UCSD's Center for Astrophysics and Space Science, presented their findings today at a meeting of the American Astronomical Society (AAS) in Winston-Salem, North Carolina.
To help with their studies, the researchers aimed the Keck I Telescope, located in Hawaii, at what they believe are protogalaxies that existed when the Universe was less than 20 percent of its current age.
These clouds of cool hydrogen are believed to evolve into galaxies when the hydrogen they contain collapses under the pull of gravity to form stars. The disks of gas, which rotate like a giant pinwheel, are believed to have given birth to galaxies about one billion years after the Big Bang.
Scientists spent more than 15 years trying to detect the faint starlight emitted by proto-galactic disks deep in space, with but few successes. But Wolfe, Prochaska and other scientists instead focused on the gas within the protogalaxies and found that they could be detected by examining how they absorb light emitted by very bright quasars behind them.
"The quasars act like flashlights for us to observe the Universe between us and the quasar," said Prochaska. "What we do is look for ways in which light from the quasar has been altered by the stuff that lies between us and the quasar, in this case, gas."
To distinguish protogalaxies from other objects, the UCSD researchers focused on a sample of 30 objects which display a characteristic signature in the absorption spectrum called a damped Lyman alpha line--which is created by neutral hydrogen in these gas clouds. Prochaska and Wolfe then used Keck I's high-resolution spectrometer, HIRES, to analyze absorption features produced by heavy elements such as silicon, nickel, and iron.
By studying such absorption lines in these 30 gas clouds, the scientists were able to determine how fast the disks rotate. Elements that are part of the disk rotating away from Earth will be shifted toward the red end of the spectrum by the Doppler effect, while those moving toward the observer will be shifted toward the blue end of the spectrum. (The Doppler effect is the apparent change in the frequency of a wave, such as a light wave or sound wave, resulting from the relative motion of the source and the receiver.)
Among other things, the scientists noted a "systematic asymmetry" in the resulting absorption profiles, indicating that the greatest densities of gas are moving at either the highest or lowest velocity. They also learned that the protogalaxies were rotating up to 300 kilometers per second. Such speeds are far faster than previously predicted.
"Low rotation speeds mean low mass, and that's what was predicted," said Wolfe. "For example, it was predicted that most objects would be rotating slower than 100 kilometers per second. So, it's a big problem."
Subsequent computer models ruled out other possible physical explanations for these results, such as dwarf galaxies, or gas within halos of dark matter. Most importantly, their simulations ruled out low mass protogalaxies, whose mergers would result in the creation of modern galaxies.
Wolfe added that a new technique allowed the researchers to distinguish different parts of the galaxy. For example, they discovered that the inner regions of these disks are more metal enriched than the outer regions.
Wolfe believes that the metals in the enriched inner regions of the thick disks of these galaxies are a byproduct of star formation in the same sector of the galaxy. As these stars evolve over time, they eject more metal-enriched gas, which ultimately cools and collapses to form a thin disk.
"The process, combined with possible radial contraction of the outer thick disk, may form the thin disks we see today," said Wolfe. "This is pure speculation, but that's the picture that we think is emerging."
The research was supported by grants from the National Science Foundation and NASA.
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