Cosmic explosion detonates in empty space

 作者:壤驷哙     |      日期:2019-02-27 09:12:04
By Maggie McKee (Image: B Cenko et al/W M Keck Observatory) (Image: NASA/H Ford et al) Astronomers are puzzling over a powerful cosmic explosion that seems to have detonated in a region of empty space, far away from any nearby galaxy. It may have been the death cry of a star that was born from debris strewn out of a past galactic dustup. Six spacecraft around Earth and Mars detected a powerful volley of gamma rays lasting about a minute on 25 January 2007. Such explosions, called long gamma-ray bursts, are thought to be caused when massive stars explode and their cores collapse into black holes. But follow-up observations by some of the world’s most powerful telescopes failed to turn up any sign of a ‘host’ galaxy for the dying star. Spectral observations did show, however, that the burst, called GRB 070125, had exploded within a small pocket of dense gas. “We tried to come up with some way in which we could imagine there could be this dense, small-scale sort of environment, but without any sort of big galaxy it’s associated with,” says Brad Cenko of Caltech in Pasadena, California, US, lead author of a study on the observations. He and his colleagues realised that galaxies that are moving past each other or merging can strip gas away from each other. This gas can get pulled into long streams, called tidal tails, where it can create dense knots of star formation. “Even if the galaxies have stopped forming stars, in the tidal tails you can trigger new episodes of star formation,” Cenko told New Scientist. Indeed, the team found two galaxies that may have swerved by each other in the past, creating a tidal tail that may have given rise to GRB 070125. Although their distance from Earth is not clear, they appear to lie 89,000 and 150,000 light years from the burst, respectively. That is relatively far away, but stars in the tidal tails of other galaxies, such as the Tadpole (shown below right), can reside even farther away from their galactic parents. “Maybe [the two galaxies] have interacted already,” says Cenko. “Sometime in the past, they’ve strewn this debris and star formation out to large distances from the centre of these galaxies, and maybe that’s where this GRB is coming from.” The researchers hope to use the Hubble Space Telescope’s keen eyesight to observe the burst’s surroundings. They will look for a host galaxy that may have been too faint for ground-based telescopes to have detected. And they will search for faint wisps of matter streaming from surrounding galaxies – a sign that those galaxies may have interacted gravitationally in the past. Journal reference: