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| Space on Apr 09, 2011

Galaxy collisions cause black holes to merge and devour nearby stars

When galaxies collide and black holes merge, the resulting monster black holes may go on a rampage, devouring nearby stars.

New research by astronomers indicates that – when two galaxies collide in space – the collision causes the black holes at their cores to spiral toward each other, merge, then go on a rampage of star-eating.

In other words, the galaxy and collision and black hole merger kicks the resulting monster black hole into surrounding stars. There, the black hole shreds and swallows stars rapidly. This research – by Nick Stone and Avi Loeb of Harvard-Smithsonian Center for Astrophysics – suggests that sky surveys might offer astronomers a way to catch a gorging black hole “in the act.”

In the artist’s conception below, two black holes are about to merge. When they combine, these astronomers believe that gravitational wave radiation will “kick ” the black hole like a rocket engine, sending it rampaging through nearby stars.

Artist's concept for black hole merger. Credit: David A. Aguilar (CfA)

Before the merger, as the two black holes whirl around each other, they stir the galactic center like the blade 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 black hole in the opposite direction like a rocket engine. Stone said in a press release:

That kick is very important. It can shove the black hole toward stars that otherwise would have been at a safe distance. Essentially, the black hole can go from starving to enjoying an all-you-can-eat buffet.

When tidal forces rip a star apart, its remains will spiral around the black hole, smashing and rubbing together, heating up enough to shine in the ultraviolet or X-rays. 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 center. A stationary black hole disrupts one star every 100,000 years. In the best-case scenario, a wandering black hole could disrupt a star every decade. This would give astronomers a much better opportunity of spotting these events.

Catching the signal 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.

Gravitational wave measurements yield very accurate distances (to better than one part in a hundred, or 1 percent). However, they don’t provide precise sky coordinates. A star’s tidal disruption will let astronomers pinpoint the galaxy containing the recently merged black-hole binary.

By correlating the galaxy’s redshift (a change in its light that’s 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. Loeb said:

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.

Finding a merged black hole also would allow theorists to explore a new regime of Einstein’s general theory of relativity. Loeb added:

We could test general relativity in the regime of strong gravity with unprecedented precision.

Bottom line: Nick Stone and Avi Loeb of Harvard-Smithsonian Center for Astrophysics have conducted new research showing that galaxy collisions, and subsequent mergers of the black holes in the centers of galaxies, would cause the newly formed monster black holes to “go on a rampage” of shredding and devouring nearby stars. As the stars are eaten by the black hole, the black hole will glow brightly in X-ray or ultraviolet radiation, giving astronomers a chance to glimpse them.

Via EurekAlert


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