Astronomers can’t send a telescope billions of light-years into space to take close-ups of the most remote galaxies, but they appear to have done the next best thing. Researchers say they’ve found a class of galaxies in our cosmic backyard that are nearly identical to some faraway ones.
By studying the easily observed nearby population, astronomers may have a novel tool for probing the long-ago era during which the first starlit bodies formed, notes Tim Heckman of Johns Hopkins University in Baltimore. He and his colleagues, including Roderik Overzier of Johns Hopkins, report their findings online (http://xxx.lanl.gov/abs/0709.3304).
The team first used NASA’s GALEX observatory to find nearby galaxies with the same pattern of ultraviolet light as the distant population known as Lyman break galaxies. Most of the roughly 2,000 members of this distant class lie between 9 billion and 12 billion light-years away.
After identifying several candidates with GALEX and studying them further with NASA’s Chandra X-ray Observatory, Heckman’s team cast the sharp eye of the Hubble Space Telescope on some of the nearby galaxies, which reside 1 billion to 2 billion light-years from Earth. Blurring the images to simulate what the galaxies would look like if they were much farther away, the team found that eight of them matched the shapes of Lyman break galaxies.
Like the Lyman break type, the nearby galaxies are producing stars at a prodigious rate. Both the near and the faraway galaxies are small, have similar masses, and have low abundances of elements heavier than helium. The presence of streams and tails suggests that the nearby galaxies came into being and forged most of their stars during collisions between smaller, gas-rich galaxies. That strongly suggests that the Lyman break galaxies, which lie too far away for tails and streams to be detected, formed in the same way, Heckman says.
“Our results are the most direct confirmation to date of models that predict that the main mode of star formation in the early universe was highly collisional,” the team claims.
The Hubble images show that the eight nearby galaxies pack star birth into several knots or clusters. The finding suggests that the Lyman break galaxies also contain knots, though these features would be too small and far away to be seen. Such dense clusters of stars would ultimately coalesce to form supermassive black holes. These star clusters may be the seeds of some of the universe’s first giant black holes, Heckman speculates.
He and his colleagues now plan to use Hubble to determine whether an additional two dozen or so nearby galaxies are also twins of the Lyman break type.
But even as many as 30 nearby galaxies could be too small a sample to reveal the true nature of the Lyman break galaxies, says Alice Shapley of Princeton University. She notes that exploring the neighborhoods of the local galaxies will be crucial for gauging how well the starbursts match the Lyman break galaxies. Just because some galaxies today have the same mass and size as others had in the distant past doesn’t mean that they’ll develop as the earlier ones did, she cautions.
