Apparent Concentrations of Galaxies Puzzles Astronomers

New observations of galaxies reveal perplexing concentrations in certain directions, astronomers said today.

Galaxies along the sight-lines toward distant explosive gamma-ray bursts appear to be four times as abundant as in the directions of quasars.

Gamma-ray bursts are massive eruptions of dying stars. Quasars are constantly bright objects are thought to involve supermassive black holes surrounded by developing galaxies.

There is no known reason why foreground galaxies should have any association with these background light sources, researchers said.

"The result contradicts our basic concepts of cosmology, and we are struggling to explain it," said Jason X. Prochaska, associate professor of astronomy and astrophysics at the University of California, Santa Cruz.

The findings are being debated and the end result could involve an oddity of observations rather than anything unusual about the universe.

Prochaska and graduate student Gabriel Prochter led the survey, which used data from NASA's Swift satellite to obtain observations of the transient, bright afterglows of long-duration gamma-ray bursts (GRBs). They described their findings in a paper submitted for publication in Astrophysical Journal Letters. It has yet to be published but a version posted online is being debated.

The survey involved finding galaxies that can't necessarily be seen by conventional observations.

When light from a gamma-ray burst or a quasar passes through a foreground galaxy, the absorption of certain wavelengths of light by gas associated with the closer galaxy creates a characteristic signature in the spectrum of light from the distant object. This provides a marker for the presence of a galaxy in front of the object, even if the galaxy itself is too faint to observe directly.

Prochter and Prochaska analyzed 15 gamma-ray bursts and found evidence for galaxies in front of 14 of them. Previous analysis of extensive quasar data from the Sloan Digital Sky Survey yielded far fewer intervening galaxies.

The probability that the results are a statistical fluke is less than about one in 10,000, Prochaska said.

Clearly, however, something is going on.

The researchers put forth three possibilities:

• Dust could obscure some quasars from even being seen, which would mean there are more quasars with dusty galaxies in front of them but they have not been found.
• The apparent signatures of galaxies in front of gamma-ray bursts might instead be signs of gas emitted by the bursts themselves.
• An intervening galaxy might act as a gravitational lens, enhancing the brightness of the background object, and perhaps this effect is somehow different for bursts than for quasars.

But none of these possible solutions are likely, the researchers say.

"A lot of people have been scratching their heads, and most hope that it goes away," Prochaska said. "The GRB sample is small, so we would like to triple or quadruple the number in our analysis. That should happen during Swift's extended mission, but it will take time."

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