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New population of faint protogalaxies discovered

Astronomers have found a new population of faint protogalaxies by taking the most sensitive spectroscopic survey ever of a time when the universe was only 15% of its present age. These objects are the probable building blocks of galaxies today, including our own Milky Way. The discovery substantiates a popular theory of galaxy formation. The research, conducted by Michael Rauch and George Becker of the Carnegie Observatories with colleagues, will be published in the March 1, 2008, issue of the Astrophysical Journal.

“The farther we look back into space the farther we see back in time,” explained Rauch.” We were actually trying to measure a faint signal from intergalactic gas caused by the cosmic ultraviolet background radiation. But as often happens in science, we got a surprise and found something we weren't looking for—dozens of faint, discrete objects emitting radiation from neutral hydrogen in the so-called Lyman alpha line, a fundamental signature of protogalaxies.”

The team used the European Southern Observatory's Very Large Telescope, for an unprecedented 92 hours, to expose a spectrum of the universe when it was only 2 billion years old. Most astronomers believe that when the universe was young it was filled with a thin, almost uniform gas. A popular theory of galaxy formation predicts that the gas accreted forming smaller protogalaxies, which then collided and merged to become the massive galaxies seen today. The new discovery lends strong support to this theory.

During the 1990s there was mounting evidence in favor of this hierarchical picture of galactic evolution, including measurements of distant quasars by Rauch and collaborators that showed how the properties of cosmic gas clouds—the reservoir of matter for galaxy formation—fit within that scheme.

“Most of those gas clouds are dark and visible only as foreground objects, which cast something of a shadow against a bright background quasar,” Becker said. “Intriguingly, one class of these shadows—known as damped Lyman alpha systems—was suspected to arise when those small, protogalactic building blocks intersect the line-of-sight to the quasar. For many years, these shadows were our only hint that a population of numerous early galaxies existed.”

Until now this possibility could not be tested because these protogalaxies, with their low masses and tiny stellar populations, were too faint for observations. The weak light signal that the team has now detected from these objects implies low star formation rates and a still small amount of chemical enrichment, as expected for young galaxies. The objects are about 20 times more common than all the distant galaxies ever seen from ground-based surveys, a finding consistent with the properties of the puzzling damped Lyman alpha shadows and with the abundance of early low-mass protogalaxies in the hierarchical picture.

Michael Rauch | EurekAlert!
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