Astronomers have discovered a distant pair of titanic black holes on a collision course.

The mass of each black hole is more than 800 million times that of our sun. As the two gradually approach each other in a death spiral, they will begin to send gravitational waves rippling through space-time. These cosmic ripples will add to the as-yet undetected background noise of gravitational waves from other supermassive black holes. Even before thecollision intended, the gravitational waves emanating from the pair of supermassive black holes will dwarf those previously detected from mergers of much smaller black holes and neutron stars.

"Collisions between massive galaxies create some of the most extreme environments we know, and should theoretically culminate in the meeting of two supermassive black holes, so it was incredibly exciting to find such an energetic and close pair of black holes in our Hubble Space Telescope images," said Andy Goulding, a research associate in astrophysical sciences atPrinceton who is the lead author of a paper published July 10 in Astrophysical Journal Letters.

"Binary supermassive black holes produce the highest gravitational waves in the universe," said co-discoverer and co-author Chiara Mingarelli, associate research scientist at the Center for Computational Astrophysics at the Flatiron Institute in New York. Gravitational waves from pairs of supermassive black holes "are a million times higher than those detected by LIGO."

"When these supermassive black holes merge, they will create a black hole hundreds of times larger than the one at the center of our own galaxy," said graduate student Kris Pardo, co-author of the paper.

The two supermassive black holes are especially interesting because they are about 2.5 billion light-years away from Earth. Since looking at distant objects in astronomy is like looking back in time, the pair belong to a universe 2.5 billion years younger than our own. Coincidentally, that's about the same amount of time that astronomers estimate theit will take black holes to start producing powerful gravitational waves.

In the current universe, black holes are already emitting these gravitational waves, but even at the speed of light the waves won't reach us for billions of years. However, the pair is still useful. Their discovery could help scientists estimate how many nearby supermassive black holes are emitting gravitational waves that we could detect right now.

Detecting the background of the gravitational wave would help answer some of astronomy's biggest unknowns, such as how often galaxies merge and whether pairs of supermassive black holes merge, or whether they get stuck in a nearly endless waltz around each other.

"It's a major embarrassment to astronomy that we don't know whether supermassive black holes merge," said Jenny Greene, professor of astrophysical sciences at Princeton and co-author of the paper. "For everyone in black hole physics, remarkably this is a long-standing puzzle that we need to solve."

Supermassive black holes can contain millions or even billions of suns in mass. Almost all galaxies, including our own Milky Way, contain at least one of these giants at their core. When galaxies merge, their supermassive black holes meet and begin to orbit each other. Over time, this orbit tightens as gas andstars pass between black holes and steal energy.

Once supermassive black holes get too close, however, this energy theft virtually stops. Some theories suggest that they stop about 1 parsec away (approximately 3.2 light-years). This deceleration lasts almost indefinitely and is known as the "final parsec problem." In this scenario, only very rare groups of three or more black holessupermassives result in mergers.

Astronomers can't just look for stagnant pairs, because long before black holes are separated by a parsec, they are too close to distinguish two separate objects. Moreover, they don't produce strong gravitational waves until they overcome the final obstacle of the parsec and get closer. (Observed as they were 2.5 billion years ago, supermassive black holesnewly discovered appear about 430 parsecs away).

If the ultimate parsec problem is not a problem, then astronomers expect the universe to be filled with the clamor of gravitational waves from pairs of supermassive black holes in the process of merging. "That noise is called the gravitational wave background, and it's a bit like a chaotic chorus of crickets singing at night," Goulding said. "You can't discern one cricket from another, but the volumeof noise helps you estimate how many crickets are out there."

If two supermassive black holes collide and combine, the collision will send out a loud "hiss" that will diminish the background chorus - but it is no easy task to "hear" it.

The telltale gravitational waves generated by supermassive black hole mergers are outside the frequencies currently observable by experiments such as LIGO and Virgo, which have detected the mergers of much smaller black holes and neutron stars. Scientists searching for the largest gravitational waves from supermassive black hole collisions rely on special star arrayscalled pulsars, which act like metronomes, sending radio waves at a constant rate. If a passing gravitational wave stretches or compresses the space between Earth and the pulsar, the rate will be slightly off.

Detecting the gravitational wave background using one of these pulsar time arrays requires patience and an abundance of monitored stars. The rhythm of a single pulsar can be interrupted for only a few hundred nanoseconds over the course of a decade. The louder the background noise, the larger the time interruptions and the faster the detection.

Goulding, Greene, and the other observational astronomers on the team detected the two titans with the Hubble Space Telescope. Although supermassive black holes are not directly visible through an optical telescope like Hubble, they are surrounded by bright clusters of luminous stars and hot gas attracted by the powerful gravitational pull. For its time in history, the galaxywhich houses the newly discovered pair of supermassive black holes "is basically the brightest galaxy in the universe," Goulding said. What's more, the galaxy's core is shooting out two unusually colossal gas clouds. When they pointed Hubble at it to discover the origins of its spectacular gas clouds, researchers found that the system contained not one, but two enormousblack holes.

The observational astronomers then teamed up with gravitational wave physicists Mingarelli and Pardo to interpret the discovery in the context of the gravitational wave background. The discovery provides an anchor point for estimating how many merging supermassive black holes are within sensing distance of Earth. Previous estimates relied on computer models of how many timesgalaxies merge, rather than actual observations of pairs of supermassive black holes.

Based on the data, Pardo and Mingarelli predicted that, in an optimistic scenario, there are about 112 nearby supermassive black holes that emit gravitational waves. The first detection of the gravitational wave background from supermassive black hole mergers should therefore occur in the next five years or so. If such a detection is not made, this would be evidence that theparsec's final problem may be insurmountable. The team is currently looking at other galaxies similar to the one harboring the newly discovered binary supermassive black hole. Finding additional pairs will help them further refine their predictions.

"This is the first example of a close pair of supermassive black holes we've found, but there may well be additional undiscovered binary black holes," said co-author Professor Michael Strauss, associate chair of Princeton's Department of Astrophysical Sciences. "The more we can learn about the black hole population and fusion, the better we will understand the process ofgalaxy formation and the nature of the gravitational wave background."

SOURCE / Princeton University / DOI: 10.3847/2041-8213/ab2a14