Astrophysics

Supermassive Black Hole Booted From Home by Einstein's Waves, Research Suggests

Astronomers suspect that gravitational waves, first theorized by Albert Einstein, kicked the rogue black hole out of its galactic core after the collision of two galaxies.

<p>Credit: NASA/ESA/M. Chiaberge (STScI & JHU)</p>

Astronomers have discovered a supermassive black hole that may have been evicted from the center of its host galaxy by gravitational waves triggered by the merger of two hefty precursor black holes.

Black holes are objects so dense with matter that light and other electromagnetic radiation cannot escape their gravitational fields. But the objects leave telltale footprints on surrounding gas, stars, and galaxies.

Astronomers calculate that the energy required to unseat the rogue black hole from the center of its galaxy, located about 8 billion light years from Earth, would have been equivalent to 100 million supernovas exploding concurrently. They suspect that gravitational waves, triggered by the merger of two supermassive but smaller black holes, set the stage for the black hole's expulsion.

The discovery of the newly found black hole, which has more mass than a billion suns, is the best evidence to date that supermassive black holes can also merge, just like their smaller, stellar-mass cousins.

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"When I first saw this, I thought we were seeing something very peculiar," said team leader Marco Chiaberge of the Space Telescope Science Institute (STScI) and Johns Hopkins University in a press release. His research will be published next week in the journal Astronomy & Astrophysics.

"When we combined observations from Hubble, the Chandra X-ray Observatory, and the Sloan Digital Sky Survey, it all pointed towards the same scenario," he added. "The amount of data we collected, from X-rays to ultraviolet to near-infrared light, is definitely larger than for any of the other candidate rogue black holes."

iAn llustration showing how gravitational waves can propel a black hole from the center of a galaxy. Credit: NASA/ESA/A. Feild (STScI)

The first detection of gravitational waves was made last year by the Laser Interferometer Gravitational-Wave Observatory, which picked up the distinctive vibration of two sun-sized black holes merging into a larger black hole. The waves, proposed by Albert Einstein more than 100 years ago, are similar to the ripples that form when a heavy rock is tossed into a pond.

Analysis of data collected by the Hubble and Chandra space telescopes and the Sloan Digital Sky Survey showed a bright quasar located far from its galaxy's core. Quasars are extremely bright, distant objects that are fueled by otherwise undetectable black holes.

"I was anticipating seeing a lot of merging galaxies, and I was expecting to see messy host galaxies around the quasars, but I wasn't really expecting to see a quasar that was clearly offset from the core of a regularly shaped galaxy," Chiaberge said. "Black holes reside in the center of galaxies, so it's unusual to see a quasar not in the center."

The astronomers found that the newly found black hole has traveled more than 35,000 light years from the center of its galaxy. It continues to zip away at a rate of some 4.7 million miles an hour.

At that speed, it could make the 240,000-mile trip between Earth and the moon in about three minutes. If the rate holds, the black hole will escape its host galaxy entirely in another 20 million years.

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Chiaberge and his team theorize that the merger of two galaxies set the stage for the black hole's expulsion. They suggest that as two galaxies merged, their respective black holes settled into the center of a newly formed elliptical galaxy, creating ripples across spacetime known as gravitational waves.

The suspect the black holes were different sizes and were spinning at different rates, which caused the rippling of space to be stronger in one direction. When the two objects finally collided, the gravitational waves stopped, causing the newly created behemoth to recoil and shoot off in the opposite direction of the strongest waves.

"This asymmetry depends on properties such as the mass and the relative orientation of the back holes' rotation axes before the merger," Colin Norman, an astrophysicist with the Space Telescope Science Institute and Johns Hopkins University, said in a statement. "That's why these objects are so rare."

Future observatories may one day be able to detect gravitational waves from supermassive black hole mergers and other higher-energy phenomenon.

Photo: A runaway quasar fleeing from its galaxy's central hub. A quasar is the visible, energetic signature of a black hole. Black holes cannot be observed directly, but they are the energy source at the heart of quasars.