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Ordinary Galaxy Hints at Less Ordinary Matter

18 March 2002 (All day)
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Just your average galaxy. Observations of galaxy cB58 (arrow) hint that the universe has less ordinary matter than astronomers thought.

An alien civilization curious about the lifestyle of humans shouldn't limit itself to studying celebrities. Likewise, for astronomers interested in the early universe, focusing on just the flashiest quasars--the luminous cores of active galaxies--might lead to skewed results. But so far these have been the only objects in the distant universe bright enough for detailed spectroscopic studies. Now, three Italian astronomers have recorded the spectrum of light emitted by a "normal" galaxy some 12 billion light-years away. Their results suggest that current estimates of the amount of ordinary matter in the universe may be too high.

Sandra Savaglio of the Observatory of Rome and Nino Panagia and Paolo Padovani of the Space Telescope Science Institute used the UVES spectrograph at the European Very Large Telescope in Chile to observe a galaxy known as cB58. Getting a high-resolution spectrum was possible thanks to a fortunate trick of nature: The light of the distant object is magnified 50 times by the gravity of a galaxy cluster halfway between the Earth and cB58. In the spectrum, the team found evidence of a large concentration of neutral hydrogen clouds close to the galaxy, indicating the presence of a giant cluster of embryonic galaxies.

If clouds of hydrogen also cluster around quasars--which convert all nearby neutral hydrogen to invisible ionized gas--then quasars must have ionized more hydrogen than astronomers had assumed, Savaglio says. This, in turn, means that the universe's diffuse background of ultraviolet radiation plays a smaller role in the ionization process. But calculations of the amount of ionized hydrogen in intergalactic space depend on the ionization rate for the background radiation, and a lower ionization rate would mean less ionized hydrogen.

Koen Kuijken of the University of Groningen, the Netherlands, says the results by Savaglio and her colleagues suggest that astronomers have overestimated the total amount of baryonic matter--"normal" matter, consisting of atoms. "It would change the balance of baryonic versus dark [nonbaryonic] matter, and all models of the formation of stars, galaxies, and heavy elements," he says. "But this would need much more data to be confirmed. We should be cautious and wait to see if the same result is confirmed in other lines of sight."

Related sites
Paper by Savaglio et al.
European Southern Observatory Web site

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