A flash of news from the Hubble Space Telescope: The distant universe looks about the same in two opposite directions. Three years ago astronomers made a 10-day exposure of a patch of northern sky, called the Hubble Deep Field (HDF), that revealed some of the faintest and most distant galaxies. Last month they took an equally hard look at the sky near the South Pole and found similar swarms of faint galaxies. The resemblance between the two sets of pictures was a relief, confirming astronomers' assumption that the original HDF had captured a standard view of the universe.
"It was crucial for us to test that the HDF is typical of the universe" with a second line of sight, says Alex Filippenko, a galaxy expert at the University of California, Berkeley. But the new view is more than just a reprise of the first: Instruments installed on the orbiting telescope since 1995 have enabled it to harvest far more detail this time around.
One of the southern images was made with the same camera system used in 1995. Equipped with color filters, it recorded the galaxies' colors, which hold clues to their distances. The reddest galaxies, their light "redshifted" to longer wavelengths by the expansion of the universe, are likely to be the most distant. Whereas one of the southern images was taken with the 1995 camera system, a second field was made with the newer NICMOS infrared camera and may have captured even more distant galaxies, with their light stretching into the infrared. And a third field broke light from the early universe into spectra that may yield new details about galaxy formation.
This third field could help scientists understand how galaxies formed from clouds of gas when the universe was just a few billion years old. It is centered on a quasar, a brilliant young galaxy at the far edge of the visible universe, which acts as a searchlight to reveal gas clouds along the line of sight to Earth. Astronomers are now working to measure the exact distances of the Southern Fields galaxies from ground-based telescopes in Chile, so that they can compare their distribution with that of the gas clouds. "This is going to provide an extremely important way to test our ideas of how the intergalactic medium turned into galaxies," says Robert Williams, former director of the Space Telescope Science Institute in Baltimore, Maryland.