A GALAXY’S CORE IS A BUSY PLACE, crowded with stars swarming around an enormous black hole. When galaxies collide (as they sometimes do), it gets even messier as the galaxies’ black holes spiral toward each other, merging to form an even bigger gravitational monster.
Once it is formed, the monster goes on a rampage, zooming into the surrounding starfields. There it finds a hearty meal, shredding and swallowing stars at a rapid rate.
According to new research by Nick Stone and Avi Loeb (Harvard-Smithsonian Centre for Astrophysics), upcoming sky surveys might offer astronomers a way to catch one of these gorging black holes “in the act.”
Before the merger, as the two black holes whirl around each other, they stir the galactic centre like the blades of a blender. Their strong gravity warps space, sending out ripples known as gravitational waves.
When the black holes merge, they emit gravitational waves more strongly in one direction. That inequality kicks the new black hole into motion in the opposite direction like a rocket engine.
“That kick is very important. It can shove the black hole toward stars that otherwise would have been at a safe distance,” said Stone.
“Essentially, the black hole can go from starving to enjoying an all-you-can-eat buffet,” he added.
Spotting a dying star
When tidal forces rip a star apart, its remains spiral around the black hole, smashing and rubbing together, heating up enough to shine in the ultraviolet or X-rays. This region immediately surrounding the black hole will glow as brightly as an exploding star, or supernova, before gradually fading in a distinctive way.
Importantly, a wandering, supermassive black hole is expected to swallow many more stars than a black hole in an undisrupted galactic centre.
A stationary black hole disrupts one star every 100,000 years. But in the best-case scenario, a wandering black hole could disrupt a star every decade.
Astronomers would have a much better change of spotting the latter events, particularly with new survey facilities like Pan-STARRS and the Large Synoptic Survey Telescope.
The siren call of gravity
Catching the dying scream from a disrupted star is a good start. However, astronomers really want to combine that information with gravitational wave data from the black hole merger.
The Laser Interferometer Space Antenna (LISA), a future mission designed to detect and study gravitational waves, could provide that data.
Gravitational wave measurements can potentially yield very accurate distance measurements (to better than one part in a hundred, or 1 percent) to the scene of the black hole crime. However, they won’t be able to provide precise sky direction co-ordinates.
Spotting a star’s tidal disruption will let astronomers pinpoint the galaxy containing the recently merged black-hole binary, thus providing the direction.
By correlating the host galaxy’s redshift (a change in its light caused by the expanding universe) with an accurate distance, astronomers can infer the ‘equation of state’ of dark energy.
In other words, they can learn more about the force that’s accelerating cosmic expansion, and which dominates the cosmic mass/energy budget today.
“Instead of ‘standard candles’ like supernovae, the black hole binary would be a ‘standard siren.’ Using it, we could create the most accurate cosmic ‘ruler’ possible,” stated Loeb.
Adapted from information issued by CfA. Black hole artwork by David A. Aguilar (CfA).
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