The early universe, once thought to be sedate, was a rollicking place, two observations recently reported on the Web suggest. One group of astronomers has found frenzied star birth almost as far as they could see out into space and back in time. Another has evidence that a tremendous flash of gamma rays, among the brightest of these events ever recorded, may have originated at an even greater distance.
To learn how the rate of star formation has changed back to the universe's infancy, Charles Steidel of the California Institute of Technology and his colleagues began by estimating the distances of about 1500 galaxies observed with several telescopes. Interstellar gases screen out the shortest wavelengths of light from distant galaxies. By estimating how much the expansion of the universe had displaced each galaxy's wavelength cutoff, Steidel was able to gauge its redshift--a measure of distance. Then, by recording the amount of ultraviolet light from hot young stars in the galaxies, Steidel worked out the star formation rate.
"We were surprised by what we found," says Steidel, who reported the results on the Los Alamos preprint server (http://xxx.lanl.gov). The rate did not decline for as far back as they could see--out to a redshift of more than 4, corresponding to roughly 13 billion years ago, about 90% of the way back to the big bang. "It's just a long plateau," he says. That conclusion conflicts with an earlier one based on the Hubble Deep Field (HDF) image, a 10-day space-telescope exposure made 3 years ago, which probed as far as possible in a tiny speck of the sky. The HDF analysis found a steep decline in star formation at distances greater than a redshift of 3, but the researchers involved--who include Steidel--now think that the HDF may have probed an unusually vacant region of the distant universe.
The second hint of an active young cosmos came when Andrew Fruchter of the Space Telescope Science Institute posted a paper on the Los Alamos site suggesting that a gamma ray burst (GRB) seen on 29 March originated at a redshift of about 5. GRBs--mysterious, seconds-long flashes of gamma rays--often have "afterglows" at optical wavelengths, like the embers of a campfire, whose redshifts can be found. Fruchter studied the optical afterglow of the 29 March event and concluded that it probably originated at a redshift of 5, trouncing previous GRB distance records.
If GRBs are connected with the violent deaths of the massive, short-lived stars found in star-forming regions, as some researchers have suggested, then Fruchter's distant event "is totally consistent with what we're finding," says Steidel.