March 27, 2017

How is this black hole moving at 5 million miles an hour?

Sometimes, because the numbers involved in astrophysics are so huge, we end up reading them and moving on. But let's consider this for a second: A supermassive black hole is currently traveling through space at five million miles an hour.

That's some speed, when you consider that supermassive black holes are so, well, massive, that the biggest weigh the equivalent of a billion suns.

Entities that big are not often moved to such an extent by outside forces. But Hubble Space Telescope images show that eight billion light-years from Earth in a galaxy called 3C 186, a supermassive black hole has been forced to travel, at the insane speeds described above, by gravitational waves.

Einstein first posited more than a century ago that gravitational waves are ripples in space-time caused by the most cataclysmic events in the universe.

The effect is similar to the circles that form on water after a stone lands on it.

In 2016, researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves produced when two black holes merged, proving the phenomenon.

In an upcoming paper in the journal Astronomy & Astrophysics, Space Telescope Science Institute (STScI) and Johns Hopkins University expert Marco Chiaberge and colleagues will explain why they believe gravitational waves from a pair of colliding black holes came to have an impact on the supermassive black hole in 3C 186.

When galaxies collide

The supermassive black hole that's currently rocketing through space was first spotted in a Hubble image that showed a very bright spot, a quasar, in galaxy 3C 186.

A quasar is the nucleus of a galaxy, and its brightness occurs due to the disk of gas surrounding a black hole at its center.

However, Chiaberge noticed that on this occasion, the quasar was 35,000 light-years from the center of its galaxy.

“I thought we were seeing something very peculiar,” he said in a NASA press release.

Chiaberge asked fellow experts for observations from various other instruments, including the Sloan Digital Sky Survey's telescope in New Mexico and the Chandra space observatory. Respectively, they specialize in detecting redshift, the stretching of light seen as something travels through space, and in measuring X-rays.

So why was the black hole so far from its galaxy's center?

Researchers believe the host galaxy displayed arc-shaped features called tidal tails, which are produced by the collision of two galaxies. It seemed 3C 186 had recently merged with another system, and possibly that black holes had also merged as a result.

What happened at this stage of the process is uncertain, but it's thought the collision caused the black holes to circle each other, creating a phenomenon in which gravity waves are flung out “like water from a lawn sprinkler”, as NASA puts it.

If the black holes spin unequally or have different masses, then gravitational waves may have been greater in a certain direction.

The black hole that had just merged is likely to have recoiled from the strongest gravitational waves, causing it to fly off in the opposite direction.

“This asymmetry depends on properties such as the mass and the relative orientation of the back holes' rotation axes before the merger,” said Colin Norman of STScI and Johns Hopkins University, a co-author on the paper. “That's why these objects are so rare.”

The scientists suggest that an energy equivalent to 100 million supernovas would be needed to make a black hole like the one in this case move as quickly it is. So quickly that it could travel from Earth to the Moon in just three minutes.

The supermassive black hole could have a lonely future, as it may break free from its galaxy in around 20 million years and end up roaming alone forever.


Image credit: NASA, ESA, and M. Chiaberge/STScI and JHU