charlie poenya

NASA's Swift satellite and an international team of astronomers have found a gamma-ray burst from a star that died when the universe was only 630 million years old, or less than five percent of its present age. The event, dubbed GRB 090423, is the most distant cosmic explosion ever seen.

"Swift was designed to catch these very distant bursts," said Swift lead scientist Neil Gehrels at NASA's Goddard Space Flight Center in Greenbelt, Md. "The incredible distance to this burst exceeded our greatest expectations -- it was a true blast from the past."

At 3:55 a.m. EDT on April 23, Swift detected a ten-second-long gamma-ray burst of modest brightness. It quickly pivoted to bring its ultraviolet/optical and X-ray telescopes to observe the burst location. Swift saw a fading X-ray afterglow but none in visible light.

"The burst most likely arose from the explosion of a massive star," said Derek Fox at Pennsylvania State University. "We're seeing the demise of a star -- and probably the birth of a black hole -- in one of the universe's earliest stellar generations."
Gamma-ray bursts are the universe's most luminous explosions. Most occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets -- driven by processes not fully understood -- punch through the star and blast into space. There, they strike gas previously shed by the star and heat it, which generates short-lived afterglows in many wavelengths.
"The lack of visible light alone suggested this could be a very distant object," explained team member Edo Berger of Harvard University.
Beyond a certain distance, the expansion of the universe shifts all optical emission into longer infrared wavelengths. While a star's ultraviolet light could be similarly shifted into the visible region, ultraviolet-absorbing hydrogen gas grows thicker at earlier times. "If you look far enough away, you can't see visible light from any object," he noted.

Within three hours of the burst, Nial Tanvir at the University of Leicester, U.K., and his colleagues reported detection of an infrared source at the Swift position using the United Kingdom Infrared Telescope on Mauna Kea, Hawaii. "Burst afterglows provide us with the most information about the exploded star and its environs," Tanvir said. "But because afterglows fade out so fast, we must target them quickly."

At the same time, Fox led an effort to obtain infrared images of the afterglow using the Gemini North Telescope on Mauna Kea. The source appeared in longer-wavelength images but was absent in an image taken at the shortest wavelength of 1 micron. This "drop out" corresponded to a distance of about 13 billion light-years.

As Fox spread the word about the record distance, telescopes around the world slewed toward GRB 090423 to observe the afterglow before it faded away.

At the Galileo National Telescope on La Palma in the Canary Islands, a team including Guido Chincarini at the University of Milan-Bicocca, Italy, determined that the afterglow's so-called redshift was 8.2. Tanvir's team, gathering nearly simultaneous observations using one of the European Southern Observatory's Very Large Telescopes on Cerro Paranal, Chile, arrived at the same number. The burst exploded 13.035 billion light-years away.

"It's an incredible find," Chincarini said. "What makes it even better is that a telescope named for Galileo made this measurement during the year in which we celebrate the 400th anniversary of Galileo's first astronomical use of the telescope."

A few hours later, Tanvir's team confirmed the distance using one of the European Very Large Telescopes on Cerro Paranal in Chile.

The previous record holder was a burst seen in September 2008. It showed a redshift of 6.7, which places it 190 million light-years closer than GRB 090423.

Swift is managed by Goddard. It was built and is being operated in collaboration with Penn State University, University Park, Pa., the Los Alamos National Laboratory in New Mexico, and General Dynamics of Gilbert, Ariz., in the U.S. International collaborators include the University of Leicester and Mullard Space Sciences Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, and additional partners in Germany and Japan.

Trio of Galaxies Mix It Up

A small spiral galaxy is caught between two elliptical galaxies, in a cluster called the Hickson Compact Group 90. As the elliptical galaxies continue to stretch, they will eventually swallow the smaller galaxy.

Image Credit: NASA, ESA and R. Sharples (University of Durham)

Shooting for the Moon

NASA's Lunar Reconnaissance Orbiter, or LRO, and Lunar Crater Observation and Sensing Satellite, or LCROSS, rolled aboard their Atlas V rocket to the launch pad at Cape Canaveral Air Force Station in Florida yesterday morning in preparation for launch today. 

LRO is scheduled for a one-year exploration mission at a polar orbit of about 31 miles, or 50 kilometers, the closest any spacecraft has orbited the moon. Its primary objective is to conduct investigations to prepare for future explorations of the moon. LCROSS will search for water ice on the moon by sending the spent upper-stage Centaur rocket to impact part of a polar crater in permanent shadows. LCROSS will fly into the plume of dust left by the impact and measure the properties before also colliding with the lunar surface.

Image Credit: ULA

Thanks to the Swift satellite and several ground based optical telescopes, astronomers are learning more about so-called “dark” gamma-ray bursts, which are bright in gamma- and X-ray emissions but with little or no visible light. These dark bursts are also providing astronomers with insights on finding areas of star formation that are hidden by dust. “Our study provides compelling evidence that a large fraction of star formation in the universe is hidden by dust in galaxies that do not appear otherwise dusty,” said Joshua Bloom, associate professor of astronomy at UC Berkeley and senior author of the study, who presented his findings at the American Astronomical Society meeting in California.


Gamma-ray bursts are the universe’s biggest explosions, capable of producing so much light that ground-based telescopes easily detect it billions of light-years away. Yet, for more than a decade, astronomers have puzzled over the nature of so-called dark bursts, which produce gamma rays and X-rays but little or no visible light. They make up roughly half of the bursts detected by NASA’s Swift satellite since its 2004 launch.

The study finds that most occur in normal galaxies detectable by large, ground-based optical telescopes.

“One possible explanation for dark bursts was that they were occurring so far away their visible light was completely extinguished,” said Bloom. Thanks to the expansion of the universe and a thickening fog of hydrogen gas at increasing cosmic distances, astronomers see no visible light from objects more than about 12.9 billion light-years away. Another possibility: Dark bursts were exploding in galaxies with unusually thick amounts of interstellar dust, which absorbed a burst’s light but not its higher-energy radiation.

Using one of the world’s largest optical telescopes, the 10-meter Keck I in Hawaii, the team looked for unknown galaxies at the locations of 14 Swift-discovered dark bursts. “For eleven of these bursts, we found a faint, normal galaxy,” said Daniel Perley, the UC Berkeley graduate student who led the study. If these galaxies were located at extreme distances, not even the Keck telescope could see them.

Most gamma-ray bursts occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets — driven by processes not fully understood — punch through the star and blast into space. There, they strike gas previously shed by the star and heat it, which generates short-lived afterglows in many wavelengths, including visible light.

The study shows that dark bursts must be similar, except for the dusty patches in their host galaxies that obscure most of the light in their afterglows.

The astronomers surveyed 14 bursts whose optical light was either much fainter than expected or completely absent. They found that almost every “dark” gamma-ray burst has a host galaxy that is able to be detected by large optical telescopes.

Star formation occurs in dense clouds that quickly fill with dust as the most massive stars rapidly age and explode, spewing newly created elements into the interstellar medium to seed new star formation. Therefore, astronomers presume that a large amount of star formation is occurring in dust-filled galaxies, although actually measuring how much dust this process has built up in the most distant galaxies has proved extremely challenging.

The stars thought to explode as gamma-ray bursts live fast and die young. Dark bursts may represent stars that never drifted far from the dusty clouds that formed them.

Gamma-ray bursts have been detected in infrared wavelengths as far out as 13.1 billion light-years. “If gamma-ray bursts were frequent 13 billion years ago — less than a billion years after the universe formed — we ought to be detecting large numbers of them,” explained team member S. Bradley Cenko, also at UC Berkeley. “We don’t, which indicates that the first stars formed at a less frenzied pace than some models suggested.”

The astronomers conclude that less than about 7 percent of dark bursts can be occurring at such distances, and they propose radio and microwave observations of the new galaxies to better understand how their dusty regions block light. A paper on the findings has been submitted to The Astronomical Journal.

Source: NASA, UC Berkeley, AAS