The galaxies in which two Gamma Ray Bursts have been observed have been found to be much dustier than expected, a discovery likely to force a rethink about the most powerful explosions since the Big Bang and the formation of very large stars.
Gamma Ray Bursts are followed by an afterglow, in which the area at which the explosion occurred is lit up. Often this occurs in all wavelengths of electromagnetic radiation, not just the very short lengths that give the events their name. However, in roughly half of all cases we observe so called “dark” gamma ray bursts in which gamma and X-rays are detected, but little or no visible light. These events have represented a puzzle since gamma ray bursts were first observed.
The most likely explanation has been thought to be that the dark bursts occur in galaxies thick with interstellar dust. Dust absorbs wavelengths close to visible light, but lets X-Rays and gamma rays through.
However, the theory has troubled astronomers, because it conflicts with the idea that the enormous stars thought to produce gamma ray bursts at the end of their lives are formed in regions thick with molecular gas rather than dust.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) a team from the National Astronomical Observatory of Japan have become the first to map the gas and dust in galaxies in which two bursts occurred.
"We have been searching for molecular gas in GRB host galaxies for over ten years using various telescopes around the world. As a result of our hard work, we finally achieved a remarkable breakthrough using the power of ALMA. We are very excited with what we have achieved,” said Professor Kotaro Kohno one of the authors of a paper in Nature outlining their findings.
Molecular gas itself is very hard to track at the distances involved here, 4.3 billion light years for GRB 020819B and 6.9 billion light years for GRB 051022. However, observations in closer galaxies show that the easier to observe carbon monoxide is a good proxy for molecular gas, so this was used instead.
“The bursts happened in regions rich in dust, but not particularly rich in molecular gas,” the paper reads. “The ratio of molecular gas to dust (<9–14) is significantly lower than in star-forming regions of the Milky Way and nearby star-forming galaxies, suggesting that much of the dense gas where stars form has been dissipated by other massive stars.” The difference is not subtle, in the Milky Way dust makes up about 1% of the mass of the interstellar medium, a tenth of what was found for these galaxies.
GRB 020819B proved particularly intriguing, with very dusty outskirts to the galaxy in which it occurred but molecular gas towards the center.
"We didn't expect that GRBs would occur in such a dusty environment with a low ratio of molecular gas to dust. This indicates that the GRB occurred in an environment quite different from a typical star-forming region," said team leader Bunyo Hatsukade.
While the observations certainly explain why these, and presumably many other bursts, are dark it opens up the question of how the regions came to be so dusty. The researchers propose that the sort of huge stars capable of producing gamma ray bursts change the environment around them. The intense ultraviolet light emitted by very large stars seems the most likely explanation, since this light can break the bonds within molecular gas, reducing its frequency relative to that of dust.
While such widespread changes might be too much for a single star, large stars tend to form together, so a star capable of producing a gamma ray burst is likely to have many neighbors of almost equal size.