Pair of supermassive black holes discovered on a collision course

Astronomers have spotted a distant pair of titanic black holes in the direction of a collision.

The mass of each black hole is more than 800 million times greater than that of our sun. As the two move closer together in a spiral of death, they will begin to send gravitational waves waving through space-time. These cosmic undulations will join the undetected background noise of the gravitational waves of other supermassive black holes.

Even before the planned collision, the gravitational waves emanating from the pair of supermassive black holes will surpass those previously detected when fusing black holes and much smaller neutron stars.

"The supermassive black hole binaries produce the strongest gravitational waves in the universe," said co-discoverer Chiara Mingarelli, a research associate at the Flatiron Institute's Astrophysics Center in New York. The gravitational waves of the supermassive black hole pairs "are one million times stronger than those detected by LIGO".

The study was led by Andy Goulding, associate researcher at Princeton University. Goulding, Mingarelli and colleagues at Princeton and the US Naval Research Laboratory in Washington, DC, report on the discovery on July 10 th the Letters from the Astrophysical Journal.

The two supermassive black holes are particularly interesting because they are about 2.5 billion light-years away from Earth. Like looking at distant objects in astronomy is like looking back, they belong to a universe twice as young as ours. Coincidentally, it's about the same time that astronomers believe that black holes will take to produce powerful gravitational waves.

In the current universe, black holes already emit these gravitational waves, but even at the speed of light, they will not reach us for billions of years. The duo is always useful, however. Their discovery can help scientists estimate the number of nearby supermassive black holes emitting gravitational waves that we could detect at the moment.

Detecting the background of gravitational waves will help solve some of the biggest unknowns in astronomy, such as the melting frequency of galaxies and the fact that pairs of supermassive black holes get confused or get stuck in an almost infinite waltz.

"It's a big drawback for astronomy not to know if supermassive black holes are coming together," says study co-author Jenny Greene, a professor of astrophysics at Princeton. "For all black hole physics specialists, it's a long-standing puzzle that we need to solve."

Supermassive black holes contain millions or even billions of suns. Almost all galaxies, including the Milky Way, contain at least one of the giants. When galaxies merge, their supermassive black holes meet and begin to spin around each other. Over time, this orbit tightens as gas and stars pass between black holes and steal energy.

Once the supermassive black holes are close enough, the stealing of energy stops almost completely. Some theoretical studies suggest that black holes then block at about 1 parsec (about 3.2 light-years) apart. This slowdown lasts almost indefinitely and is known as the last problem of parsec. In this scenario, only very rare groups of three or more supermassive black holes result in mergers.

Astronomers can not simply search for blocked pairs because, long before black holes are separated from a parsec, they are too close to be distinguished into two separate objects. In addition, they do not produce huge gravitational waves as long as they do not overcome the obstacle of the final parsec and do not get closer. (Observed 2.5 billion years ago, the new supermassive black holes appear at about 430 parsecs away.)

If the final parsec problem does not exist, astronomers expect the universe to be filled with the clamor of gravitational waves from pairs of supermassive black holes. "This sound is called gravitational grounding, and it's kind of like a chaotic chorus of crickets singing in the night," Goulding said. "You can not discern one cricket from another, but the volume of the noise helps you to estimate the number of locusts that exist." (When two supermassive black holes meet and combine, they send a deafening thunder that surpasses all others.) Such an event is brief and extraordinarily rare, so scientists do not expect to find one so early. )

The gravitational waves generated by pairs of supermassive black holes are outside the frequencies currently observable by experiments such as LIGO and Virgo. Instead, gravitational hunters rely on special star networks called pulsars, which act as metronomes. Fast-moving stars send out radio waves at a steady pace. If a passing gravitational wave expands or compresses the space between the Earth and the pulsar, the rhythm is slightly shifted.

Detecting gravitational backgrounds with one of these pulsar synchronization matrices requires patience and many monitored stars. The rhythm of a pulsar can be disturbed by a few hundred nanoseconds in a decade. The stronger the background noise, the greater the perturbation of synchronization and the sooner the first detection is made.

Goulding, Greene and the other team observing astronomers detected the two titans with the Hubble Space Telescope. Although the supermassive black holes are not directly visible through an optical telescope, they are surrounded by bright bunches of bright stars and hot gases attracted by the powerful gravitational tug. For its time, the galaxy hosting the new pair of supermassive black holes "is basically the brightest galaxy in the universe," Goulding said. In addition, the core of the galaxy is projecting two exceptionally large gas plumes. After pointing the Hubble Space Telescope on the galaxy to discover the origins of its spectacular gas clouds, the researchers discovered that the system contained not one but two gigantic black holes.

The observationalists then teamed up with gravitational wave physicists Mingarelli and Princeton graduate student Kris Pardo to interpret the discovery against the backdrop of the gravitational wave background. The discovery provides an anchor point for estimating how many pairs of supermassive black holes are in the Earth's detection distance. Previous estimates were based on computer models of galaxy fusion frequency, rather than actual observations of supermassive black hole pairs.

Based on the results, Pardo and Mingarelli predict that, in an optimistic scenario, there are approximately 112 supermassive black holes nearby emitting gravitational waves. The first detection of the gravitational wave background from supermassive black holes should therefore occur in the next five years or so. If such a detection is not carried out, it would be proof that the final parsec problem can be insurmountable. The team is currently looking for other galaxies similar to the one that houses the new pair of supermassive black holes. Finding additional pairs will help them refine their forecasts.

Provided by
Simons Foundation