Galaxies Undercounted in Cosmic Census

The United States wants everyone to stand up and be counted in the 2010 census, but meanwhile many galaxies have been undercounted in their own celestial census.

It turns out that about 90 percent of distant galaxies do not show up in a commonly used galaxy survey method, a new study has found.

"If there are 10 galaxies seen, there could be a hundred there," said study leader Matthew Hayes of the Observatory of the University of Geneva in Switzerland.

A common way to search for galaxies is to look for a thin range of light emitted by hydrogen within the galaxies called the Lyman-alpha line. Scientists have suggested that some of the Lyman-alpha light never makes it to our telescopes on Earth because it is absorbed by gas and dust within the distant galaxies themselves.

To test this idea, researchers conducted an extensive survey of far-off galaxies about 10 billion light-years from Earth. First, they looked for galaxies that emitted Lyman-alpha light. Then they looked at the same patch of sky through a filter that only let in another wavelength range called the H-alpha line, which is also emitted by glowing hydrogen.

The astronomers found that many of the galaxies apparent in H-alpha light were completely dark in the Lyman-alpha line, and would have been missed in a survey that only observed in Lyman-alpha light.

"Astronomers always knew they were missing some fraction of the galaxies in Lyman-alpha surveys, but for the first time we now have a measurement," Hayes said. "The number of missed galaxies is substantial."

The observations were made with the FORS and HAWK-I cameras on the European Southern Observatory's Very Large Telescope in Chile. The researchers detail their findings in the March 25 issue of the journal Nature.

While Lyman-alpha light is still a useful way of hunting for galaxies, the new findings help put these surveys in context.

"Now that we know how much light we?ve been missing, we can start to create far more accurate representations of the cosmos, understanding better how quickly stars have formed at different times in the life of the Universe," said co-author Miguel Mas-Hesse of the Centro de Astrobiologia (CSIC-INTA) in Madrid, Spain.

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