Astronomers using the Hubble Space Telescope have measured accurate distances to several faint, red galaxies seen in the Hubble Ultra Deep Field, confirming that three fourths are among the most distant galaxies yet studied. This is a milestone because the Hubble data provide spectra of objects 10 times fainter than have been studied with spectrometers on ground-based telescopes.

This allows researchers to probe the common galaxies in the early universe, which are believed to be responsible for most of the energy output at that time, and perhaps also for ionizing and heating the tenuous gas in between galaxies. Surprisingly, the distant galaxies are similar in many ways to their considerably closer descendants.

Sangeeta Malhotra, Norbert Pirzkal, Chun Xu, and James Rhoads, of the Space Telescope Science Institute in Baltimore, Md., reported these results June 2 at the American Astronomical Society's annual meeting in Denver, on behalf of the GRAPES (Grism ACS Program for Extragalactic Science) Team.

Spectroscopy is the gold standard for measuring galaxy distances. A spectrum splits up the light from galaxies into finer colors. By detecting the distinct signature of hydrogen in these galaxies, Malhotra and her colleagues are able to confirm that 16 among 22 are really at the distances their colors suggest. The confirmation is an important step because distant galaxies are not the only kind of faint red object in images like the Ultra Deep Field: Indeed, three objects turn out to be much closer galaxies reddened by dust, and three more are cool stars in the Milky Way.

"Combining spectra with the UDF images is like adding fingerprints to mug shots," says Pirzkal. "It really boosts your confidence that you have the right identification."

The team also finds that most of the galaxies, which existed when the universe was only about one billion years old, have populations of stars similar to the much closer galaxies that could be up to three billion years old. Some researchers have predicted that the earliest galaxies should be much bluer due to an abundance of extremely hot stars. But Malhotra reports this doesn't seem to be the case in this sample. Many of the galaxies also appear to be interacting, which suggests that we are witnessing their early growth phase.

The observations were made with Hubble's Advanced Camera for Surveys.