Rocky Balboa would be proud. Halfway across the Milky Way, a gas cloud has punched a hole in the galaxy, leaving a blast wave and a gaping void 3000 light years across.
Compared with our galaxy, this cloud, named CHVC040, is a lightweight: only 16,000 times the mass of the sun. It is moving at high velocity and should have disintegrated on impact with the galactic disc, according to simulations, so it’s a mystery how it created this hole.
The galaxy is dotted with these spherical abysses, called supershells. Most are thought to be caused by a series of supernova explosions, when the hottest young stars in a cluster blow themselves to bits. The expanding blast waves burst through the galaxy with an energy equivalent to dozens of such exploding stars
But many supershells have no trace of star clusters at their centre, so there might be other causes.
One suggestion was that the compact clouds of gas that swarm around the Milky Way might be jabbing into the disc from above and below, causing explosive impacts. Much larger clouds have been spotted ploughing through the galaxy, but whether the smaller ones could make a mark remained an open question. Supershells dissipate quickly, and evidence of such a collision has been hard to come by.
Until this one. A pair of South Korean astronomers spotted a supershell with no apparent cause back in 2007. But when Seoul National University graduate student Geumsook Park took a close look at new data from the highly sensitive Arecibo radio telescope in Puerto Rico, she saw a compact high-velocity cloud smack dab in the middle of the shell.
When she showed the data to Bon-Chul Koo, her adviser and one of the supershell’s discoverers, he got excited. “I knew that this was important,” he says. “I think this is the first convincing example.” Park and Koo think the odds that the cloud and the shell could appear together by chance are exceedingly low.
Understanding just how these shells form should tell us more about the lifecycle of stars in the universe, as the blast waves from supershells are thought to sweep up debris into clumps that turns atomic gas into molecular clouds, which then become the next generation of stellar nurseries.
Interpreting three-dimensional structure from radio data can be tricky, says astronomer Joanne Dawson of Macquarie University in Sydney, but she finds the results convincing. The supershell is in relatively empty space, well below the densest part of the Milky Way disc, so exploding stars are unlikely to have made it. “I pretty much bought it,” she said. “It’s really nice to see an article where they have some reasonable evidence that a cloud is smashing through the disc and forming a shell.”
Journal reference: Astrophysical Journal Letters, http://arxiv.org/abs/1607.07699
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