Going the Distance: Galaxies may hail from early universe

Using a cosmic magnifying glass to peer into the deepest reaches of space, two teams of astronomers have discovered tiny galaxies that may be among the most distant known. Images suggest that one of the galaxies is so remote that the light now reaching Earth left this starlit body when the 13.7-billion-year-old universe was only about 700 million years old.

LONG AGO, FAR AWAY. Gravity of the cluster Abell 1689 acts as a gravitational lens, bending into arcs and magnifying the light from remote background galaxies. One galaxy appears so remote that it doesn’t show up in visible light (top, right) but only in the infrared (middle and bottom, right). L. Bradley & H. Ford/JHU, Bouwens & Illingworth/UCSC, NASA, ESA

The discoveries are important, notes Tim Heckman of Johns Hopkins University in Baltimore, because they probe a special time in the universe, when the cosmos changed from a place filled with neutral gas to a place ionized by the emergence of the first substantial population of stars and black holes. Studies of distant galaxies help pinpoint when that critical era happened.

All of the galaxies are so small that even the keen eye of the Hubble Space Telescope couldn’t have spotted them without nature providing a gravitational assist. According to Einstein’s theory of general relativity, a massive foreground body acts like a lens, bending and magnifying light from a more remote galaxy that lies along the same line of sight to Earth.

That’s why Garth Illingworth and Rychard Bouwens of the University of California, Santa Cruz and their colleagues went hunting for distant galaxies around a nearby cluster of galaxies called Abell 1689.

The cluster’s gravity distorts images of background galaxies, bending them into arcs and magnifying their brightness. One of these galaxies proved especially intriguing because it appeared bright at several infrared wavelengths recorded by Hubble but disappeared in visible light.

That’s a sign that the galaxy, dubbed A1689-zD1, is both extraordinarily distant and youthful. The data also indicate that the galaxy forms stars at a rate equivalent to five suns a year, typical of the small galaxies thought to be common in the early universe, says Bouwens.

The researchers don’t have a spectrum for the galaxy and therefore can’t be sure of its distance, but they calculate in an upcoming Astrophysical Journal paper that the galaxy most likely lies 13 billion light-years from Earth and has a redshift of 7.6. That redshift signifies that cosmic expansion has stretched the wavelengths emitted by the galaxy by a factor of 8.6.

“The reason we are excited about this [galaxy] is that we can look at it in great detail because of the factor of 10 gravitational amplification by the foreground cluster,” Bouwens says. A1689-zD1 is the brightest known galaxy that’s likely to be extremely distant, his team notes.

The Hubble images show several dense clumps, each containing hundreds of millions of stars. Follow-up images, taken at longer infrared wavelengths with NASA’s Spitzer Space Telescope, provide additional evidence that the galaxy is remote and also yield a more accurate measurement of the galaxy’s mass.

“It looks pretty convincing” that A1689-zD1 is remote, but proof may require spectra taken by Hubble’s proposed successor, the James Webb Space Telescope, Heckman says.

In searching for distant galaxies, a second team, which includes Richard Ellis and Johan Richard of the California Institute of Technology in Pasadena, also surveyed several galaxy clusters. The team found evidence of six distant galaxies, which may lie between 12.9 billion and 13.1 billion light-years from Earth, Richard reported this week at an astrophysics meeting at the Aspen Center for Physics in Colorado. Because the galaxies aren’t intrinsically as bright, even though the clusters magnify them by a factor of about 10, astronomers have less information about these faint bodies than they do about A1689-zD1, Richard notes.

Correction, 2/13/2008:
This article was obscure about the brightness of one group of distant galaxies observed by a team that includes Richard Ellis and Johan Richard of the California Institute of Technology. The story implied that these galaxies appear faint because gravitational magnification of their light by foreground galaxy clusters was less than that for another distant galaxy, A1689-zD1, which is unusually (intrinsically) bright. In fact, the dimness is intrinsic, as the magnification was roughly the same for the faint group and A1689-zD1.

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