04/04/2001 07:00:00

Distant Massive Explosion Reveals a Hidden Stellar Factory

A gamma-ray burst detected in February has led astronomers to a galaxy where the equivalent of 500 new suns is being formed each year.

The discovery of a new "starburst galaxy," made by researchers from the National Radio Astronomy Observatory and the California Institute of Technology, provides support for the theory that gamma-ray bursts are caused by exploding young massive stars. Details of the discovery are being presented today at the Gamma 2001 conference.

"This is a tremendously exciting discovery, since gamma-rays can penetrate dusty veils, and thus gamma-ray bursts can be used to locate the hitherto difficult-to-study dust galaxies at high redshifts," says Fiona Harrison, assistant professor of physics and astronomy at Caltech. "Gamma-ray bursts may offer us a new way to study how stars are formed in the early universe."

Radiation from this gamma-ray burst was first detected in the constellation Bootes by the Italian satellite observatory BeppoSAX on Feb. 21. Within hours, astronomers worldwide received the news of the burst and began looking for a visible light counterpart. The burst was one of the brightest recorded in the four years BeppoSAX has been standing watch.

Gamma-ray bursts were first detected by satellites monitoring the Nuclear Test Ban Treaty in the 1970s, and were thought for many years to represent relatively modest outbursts on nearby neutron stars. The events have now been shown to originate in the farthest reaches of the universe. They produce, in a matter of seconds, more energy than the sun will generate in its entire 10-billion-year lifetime, and represent the most luminous explosions in the cosmos.

After the February event, astronomers at the U.S. Naval Observatory discovered the visible light counterpart, pinpointing the location of the event. An international collaboration, led by Dale Frail of the National Radio Astronomical Observatory and Harrison and Shri Kulkarni of Caltech, conducted a variety of observations using the Hubble Space Telescope, the Very Large Array radio telescope, the Chandra X-ray Observatory, Institut de RadioAstronomie Millimétrique (IRAM), and the James Clerk Maxwell telescope (JCMT).

Pivotal to the detection of the starburst galaxy was the latter telescope, which sits high atop Mauna Kea in Hawaii and is designed to make measurements at the shortest radio wavelengths capable of penetrating Earth's atmosphere, called the "submillimeter" portion of the spectrum. Only five and a half hours after the first sighting, a submillimeter source was found at the burst location.

Astronomers had expected to see a rapidly brightening signal with JCMT, a sign that the shock generated by the burst was moving through the dense gas surrounding the burst. Instead, much to everyone's surprise, the signal stayed constant, never varying during this time.

Furthermore, observations conducted at a slightly lower frequency by observers on the IRAM telescope in Southern Spain showed a much fainter source, strongly suggesting that the submillimeter observation was not simply detecting the afterglow of the explosion.

"The simplest explanation is that we have detected the light from the host galaxy of the burst," says Frail, explaining that it is rare to detect galaxies at submillimeter wavelengths. Only about one in every thousand that are visible with optical telescopes are observed by short-wavelength radio telescopes.

Astronomers in Arizona found the gamma-ray burst to lie roughly 8 billion light-years from Earth. This was also confirmed almost simulataneously by the Caltech group using one of the 10-meter Keck telescopes on Mauna Kea. The light we see from it shows the galaxy when the universe was less than half its present age. At this distance, the observed submillimeter brightness implies a prodigious rate of star formation—roughly 500 solar masses of material must be turning into stars each year, meaning that one or two new stars shine forth each day. The galaxy in which the burst occurred, then, may provide a glimpse of what the Milky Way looked like in its youth.

Previous searches for starburst galaxies in the distant universe have been hampered by the imprecise positions current submillimeter telescopes provide, and by the obscuring dust and gas that largely hides such galaxies from the view of optical telescopes. Observers led by Kulkarni used the Hubble Space Telescope to observe the fading embers of the explosion, but the underlying galaxy seems ordinary as seen in visible light, since most of this light is likely absorbed by dust and converted into submillimeter radiation. Had the galaxy been observed only in the optical wavelengths, astronomers would not have guessed that so many stars were being formed in it.

The discovery of a bright gamma-ray burst in a starburst system is exciting for two reasons: it strongly supports one model for the bursts themselves—the explosive destruction of a young, massive star—and it suggests a new way to locate such galaxies. With their enormous penetrating power, the energetic gamma rays punch right through the dusty veil, pinpointing the location of vigorous star-forming activity.

By following up on the hundreds of bursts that will be detected in the next few years, astronomers will be able to collect an unbiased sample of starburst galaxies at different distances in time and space, enabling them to explain the star-formation history of the universe.

The members of the Caltech team also include: Prof. S. George Djorgovski; postdoctoral fellows Derek Fox, Titus Galama, Daniel Reichart, Re'em Sari and Fabian Walter; and graduate students Edo Berger, Joshua Bloom, Paul Price, and Sarah Yost. Astronomers from several other institutions are also involved in the collaboration.

Written by Robert Tindol