Intergalactic "Shot in the Dark" Shocks Astronomers

In this dramatic Hubble Space Telescope image, we see the "tidal tail" of the Tadpole Galaxy, UGC 10214. Its blue color indicates that young, massive stars are being formed in the tail, far from the large mass of stars and gas that make up the galaxy itself. Penn State's Derek Fox and colleagues have announced their identification of a gamma-ray burst, named GRB070125, which may have occurred in a tidal tail like this one, far from any bright galaxy. However, since GRB070125 is more than twenty times as distant from Earth as the Tadpole Galaxy, it is not possible to see whether a tidal tail exists in this region of space. The Tadpole Galaxy tail is more than 300,000 light-years long. The minimum distance from GRB070125 to its host galaxy is 88,000 light-years, or approximately 60-percent of the length of the scale bar in this image. Credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA

A team of astronomers at Penn State and Caltech has discovered a cosmic explosion that seems to have come from the middle of nowhere — thousands of light-years from the nearest galaxy-sized collection of stars, gas, and dust. This "shot in the dark" is surprising because the type of explosion, a long-duration gamma-ray burst (GRB), is thought to be powered by the death of a massive star.

"Here we have this very bright burst, yet it's surrounded by darkness on all sides," says Brad Cenko of the California Institute of Technology in Pasadena, California, lead author of the team's paper, which has been accepted for publication in the Astrophysical Journal. "The nearest galaxy is more than 88,000 light-years away, and there's almost no gas lying between the burst and Earth."

The blast was detected on 25 January 2007 by several spacecraft of the Inter-Planetary Network. Observations by NASA's Swift satellite, controlled from its Mission Operation Center at Penn State, pinpointed the explosion, named GRB 070125 for its detection date, to a region of sky in the constellation Gemini. It was one of the brightest bursts of the year, and the Caltech/Penn State team moved quickly to observe the burst's location with ground-based telescopes.

Using the team's robotic 60-inch telescope at Palomar Observatory in California, the astronomers discovered that the burst had a bright and fast-fading afterglow in visible light. This discovery prompted them to observe the afterglow in detail with two of the world's largest telescopes, the 8-meter Gemini North telescope and the 10-meter Keck I telescope, both near the summit of Hawaii's Mauna Kea.

What came next was a total surprise. Contrary to experience with more than a hundred previous GRBs, the Gemini spectra revealed no signs of dense gas and dust absorbing the light of the afterglow. A trace of magnesium revealed that the burst took place more than 9.4 billion years ago, as deduced by the shift in wavelength of the afterglow's light, and that the surrounding gas and dust was more tenuous than the environment around any previous burst.

To further pin down the environment that could produce such an unusual explosion, the group obtained Keck images of the location of GRB 070125 long after its afterglow light had faded away. Surprisingly, the resulting images showed no galaxy at this location. "A Keck image could have revealed a very small, faint galaxy at that distance," says coauthor Derek Fox of Penn State.

Top left: Discovery of the optical afterglow of GRB 070125 with the robotic Palomar 60-inch telescope (taken the night of January 26, 2007). Top right: Image of the same field, taken almost a month later, with the 10-m Keck I telescope. The afterglow has faded below the sensitivity of this (much more sensitive) image, enabling a search for the galaxy in which GRB 070125 resided. Bottom: Zoom in of same Keck I image. The large white circle has the same size (280,000 light-years in radius) in all images, and the position of the afterglow is marked with a white cross. The nearest galaxy to the afterglow position lies in the foreground and is therefore unrelated to GRB 070125. Massive stars, widely believed to power long-duration GRBs like GRB 070125, are rarely found at such large distances as implied for the next two nearest objects (greater than 89,000 light-years). Credit: S. Bradley Cenko (Caltech) and Derek B. Fox (Penn State)

Astronomers have amassed a great deal of evidence that GRBs are triggered by the explosive deaths of massive stars, which live very short lives. Because of their short lifespans, massive stars don't have time to wander far from their birthplaces, usually dense clouds of gas and dust inside respectable-size galaxies. So GRB 070125 raises the perplexing question of how a massive star could be found so far away from any galaxy.

"Big stars live fast and die young, without much time to move around," says Fox. "So if this massive star died far away from any galaxy, the key question is, how did it manage to be born there?" The formation of massive stars requires similarly massive aggregations of gas and dust, which are usually found in bright galaxies.

One possibility is that the star formed in the outskirts of an interacting galaxy, as seen in the famous Hubble Space Telescope picture of the "Tadpole" galaxy, UGC 10214. "In the local universe, about one percent of star formation happens in tidal tails, on the outskirts of two interacting galaxies," says Cenko. "So it might even make sense to find one in 100 gamma-ray bursts in such an environment." If this idea is correct, it should be possible to detect the tidal tail hosting GRB 070125 by taking a long exposure with the Hubble Space Telescope. "That's definitely our next stop," says Cenko.

"Many Swift discoveries have left astronomers scratching their heads in befuddlement," adds Swift lead scientist Neil Gehrels of NASA Goddard Space Flight Center in Greenbelt, Maryland "But this discovery of a long GRB with no host galaxy is one of the most perplexing of all."

CONTACTS:

Derek Fox (Penn State): (+1)814-863-4989, dfox@astro.psu.edu

Barbara K. Kennedy (Penn State PIO): (+1)814-863-4682, science@psu.edu

Robert Naeye (NASA Goddard): (+1)301-286-4453, Robert.P.Naeye@nasa.gov

Rob Gutro (NASA Goddard): (+1)301-286-4044, Robert.J.Gutro@nasa.gov

Lynn Cominsky (Swift PIO): (+1)707-664-2655, lynnc@universe.sonoma.edu