Gamma-ray burst occurring in a dusty region

Dark gamma-ray bursts hidden by space dust

Gamma-ray bursts, the universe's most brilliant flashes of light, have revealed areas of star formation previously hidden to astronomers. The find could help astronomers understand how the universe's first stars formed and died.

Gamma-ray bursts are twin beams of radiation and light that act like camera flash bulbs, helping astronomers probe the universe. They briefly shine from Type Ic supernovas, the core collapse of massive stars that shine so brightly they strip themselves of hydrogen and helium.

Artist's illustration of a gamma-ray burst occurring in a dusty region of intense star formation. If a dust cloud lies between the burst and Earth, the optical light will be almost entirely absorbed, but the gamma-rays.

Where did the light go?

Most long-duration bursts shine for several seconds but leave an afterglow of visible light that fades over several hours. But some, so-called 'dark' gamma-ray bursts produce no visible light accompaniment to the beams of gamma rays and X-rays.

Just why these dark bursts can't be seen at optical wavelengths has puzzled astronomers. Some theorised that the bursts were so distant that their light had 'red-shifted' – that is, the light shifted out of the optical range to longer wavelengths as it crossed the expanding cosmos. This would mean that dark gamma-ray bursts came from the death throes of the universe's earliest stars.

But a new study led by astronomer Daniel Perley from the University of California, Berkeley, shows that dark gamma-ray bursts are hidden by dust in star forming regions, and not distance. He presented the research last week at the American Astronomical Society's meeting in Pasadena, California.

Not old enough

The researchers used the twin Keck 10-metre telescopes in Hawaii to follow up 29 bursts discovered by NASA's Swift gamma-ray satellite.

Fourteen of the bursts were classified as dark. For 11 of these, the researchers identified their host galaxies, none of which were further than 12.9 billion light-years away, indicating that they were not among the oldest in the universe. The other three 'dark' bursts had faint optical counterparts, indicating they also didn't come from galaxies so far back in time the optical wavelengths had shifted.

This means that dark gamma-ray bursts can reveal areas of star formation previously hidden to astronomers, in particular the dust patches in otherwise dust-free galaxies. These dust patches are difficult to see because they are hidden by brightness of the surrounding stars