Above the fading glow of a gamma ray burst, right, are fingerlike filaments from a distorted galaxy.

Ground Zero of a Cosmic Explosion

Astronomers now have their closest look yet at a distant galaxy that spawned a gamma ray burst--a titanic explosion in the distant universe. This photograph, taken by the Hubble Space Telescope 2 weeks after one of the most powerful of these mysterious events, and released yesterday, reveals a misshapen collection of stars that may be the debris of two colliding galaxies.

An instrument called the Burst and Transient Source Experiment on board the orbiting Compton Gamma Ray Observatory caught the first sight of the burst on 23 January (ScienceNOW, 25 January). Within seconds its rough coordinates were transmitted electronically to ground-based observatories around the world. And then, just 22 seconds later, the Robotic Optical Transient Search Experiment--an automated array of four 35-mm camera lenses near Los Alamos, New Mexico--snapped pictures of the patch of sky where the burst was located. The image revealed a brightening optical source--the first time an optical counterpart has been detected so soon after a burst. Even though the light source had a redshift of 1.6, putting it several billion light-years away, it was bright enough to be seen through binoculars.

In order to see the underlying galaxy, Hubble waited until early February, when the blast had faded to one four-millionth of its original brightness, to zoom in for a closeup. The photograph of the fading fireball reveals a distorted galaxy with fingerlike filaments extending above the bright white blob of the fireball. "These high-redshift galaxies can have an unusual appearance," says Andy Fruchter of the Space Telescope Science Institute in Baltimore. "So it's not clear whether we're seeing a galaxy in formation or the result of a collision between two galaxies."

The galaxy is quite blue, Fruchter says, indicating a rapid rate of star formation. So far, all of the host galaxies which have been identified for gamma ray bursts are unusually blue, which Fruchter says supports the argument that the bursts are usually associated with star formation. But Cole Miller, an astrophysicist at the University of Chicago, says there is such diversity among gamma ray bursts that it is difficult to say what any of them have in common. "This has kept alive the statement that 'If you've seen one gamma ray burst, you've seen one gamma ray burst,'" he quips.

Posted in Space