Each black hole’s mass is more than 800 million times that of our sun. As the two gradually draw closer together in a death spiral, they will begin sending gravitational waves rippling through space-time. Those cosmic ripples will join the as-yet-undetected background noise of gravitational waves from other supermassive black holes. Even before the destined collision, the gravitational waves emanating from the supermassive black hole pair will dwarf those previously detected from the mergers of much smaller black holes and neutron stars.
Titanic Twosome: A Princeton-led team of astrophysicists has spotted a pair of supermassive black holes, roughly 2.5 billion light-years away, that are on a collision course (inset). The duo can be used to estimate how many detectable supermassive black hole mergers are in the present-day universe and to predict when the historic first detection of the background “hum” of gravitational waves will be made.
Image courtesy of Andy Goulding et al./Astrophysical Journal Letters 2019
“Collisions between enormous galaxies create some of the most extreme environments we know of, and should theoretically culminate in the meeting of two supermassive black holes, so it was incredibly exciting to find such an immensely energetic pair of black holes so close together in our Hubble Space Telescope images,” said Andy Goulding, an associate research scholar in astrophysical sciences at Princeton who is the lead author on a paper appearing July 10 in Astrophysical Journal Letters.
“Supermassive black hole binaries produce the loudest gravitational waves in the universe,” said co-discoverer and co-author Chiara Mingarelli, an associate research scientist at the Flatiron Institute’s Center for Computational Astrophysics in New York City. Gravitational waves from supermassive black hole pairs “are a million times louder 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 Princeton graduate student Kris Pardo, a co-author on the paper.
The two supermassive black holes are especially interesting because they are around 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 roughly the same amount of time the astronomers estimate the black holes will take to begin producing powerful gravitational waves.
In the present-day universe, the black holes are already emitting these gravitational waves, but even at light speed the waves won’t reach us for billions of years. The duo is still useful, though. Their discovery can help scientists estimate how many nearby supermassive black holes are emitting gravitational waves that we could detect right now.
Detecting the gravitational wave background would help answer some of the biggest unknowns in astronomy, such as how often galaxies merge and whether supermassive black hole pairs merge at all, or if they become stuck in a near-endless waltz around each other.
“It’s a major embarrassment for astronomy that we don’t know if supermassive black holes merge,” said Jenny Greene, a professor of astrophysical sciences at Princeton and a co-author on the paper. “For everyone in black hole physics, observationally this is a long-standing puzzle that we need to solve.”
Supermassive black holes can contain millions or even billions of suns’ worth of mass. Nearly all galaxies, including our own Milky Way, contain at least one of these behemoths at their core. When galaxies merge, their supermassive black holes meet up and begin orbiting one another. Over time, this orbit tightens as gas and stars pass between the black holes and steal energy.
Once the supermassive black holes get too close, though, this energy theft all but stops. Some theories suggest that they stall at around 1 parsec apart (roughly 3.2 light-years). This slowdown lasts nearly indefinitely and is known as the “final parsec problem.” In this scenario, only very rare groups of three or more supermassive black holes result in mergers.
Astronomers can’t just look for stalled pairs, because long before the black holes are a parsec apart, they’re too close to distinguish as two separate objects. Moreover, they don’t produce strong gravitational waves until they overcome the final parsec hurdle and get closer together. (Observed as they were 2.5 billion years ago, the newfound supermassive black holes appear about 430 parsecs apart.)
If the final parsec problem turns out not to be a problem, then astronomers expect that the universe is filled with the clamor of gravitational waves from supermassive black hole pairs in the process of merging. “This noise is called the gravitational wave background, and it’s a bit like a chaotic chorus of crickets chirping in the night,” Goulding said. “You can’t discern one cricket from another, but the volume of the noise helps you estimate how many crickets are out there.”
If two supermassive black holes do collide and combine, it will send a thundering “chirp” that will dwarf the background chorus – but it’s no small task to “hear” it.
The telltale gravitational waves generated by merging supermassive black holes 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 hunting for the larger gravitational waves from supermassive black hole collisions rely on arrays of special stars called pulsars that act like metronomes, sending out radio waves in a steady rhythm. If a passing gravitational wave stretches or compresses the space between Earth and the pulsar, the rhythm will be thrown off slightly.
Detecting the gravitational wave background using one of these pulsar timing arrays takes patience and plenty of monitored stars. A single pulsar’s rhythm might be disrupted by only a few hundred nanoseconds over a decade. The louder the background noise, the larger the timing disruptions and the quicker the detection will be made.
Goulding, Greene and the other observational astronomers on the team detected the two titans with the Hubble Space Telescope. Although supermassive black holes aren’t directly visible through an optical telescope like Hubble, they are surrounded by bright clumps of luminous stars and warm gas drawn in by the powerful gravitational tug. For its time in history, the galaxy harboring the newfound supermassive black hole pair “is basically the most luminous galaxy in the universe,” Goulding said. What’s more, the galaxy’s core is shooting out two unusually colossal plumes of gas. When they pointed Hubble at it to uncover the origins of its spectacular gas clouds, the researchers discovered that the system contained not one but two massive black holes.
The observational astronomers then teamed up with gravitational wave physicists Mingarelli and Pardo to interpret the finding in the context of the gravitational wave background. The discovery provides an anchor point for estimating how many merging supermassive black holes are within detection distance of Earth. Previous estimates relied on computer models of how often galaxies merge, rather than actual observations of supermassive black hole pairs.
Based on the data, Pardo and Mingarelli predicted that in an optimistic scenario, there are about 112 nearby supermassive black holes emitting gravitational waves. The first detection of the gravitational wave background from supermassive black hole mergers should therefore come within the next five years or so. If such a detection isn’t made, that would be evidence that the final parsec problem may be insurmountable. The team is currently looking at other galaxies similar to the one harboring the newfound supermassive black hole binary. Finding additional pairs will help them further hone their predictions.
“This is the first example of a close pair of such massive black holes that we’ve found, but there may well be additional binary black holes remaining to be discovered,” said co-author Professor Michael Strauss, the associate chair of Princeton’s Department of Astrophysical Sciences. “The more we can learn about the population of merging black holes, the better we will understand the process of galaxy formation and the nature of the gravitational wave background.”
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Citation: “Discovery of a close-separation binary quasar at the heart of a z ∼ 0.2 merging galaxy and its implications for low-frequency gravitational waves,” by Andy D. Goulding, Kris Pardo, Jenny E. Greene, Chiara M. F. Mingarelli, Kristina Nyland and Michael Strauss, was published July 10 in Astrophysical Journal Letters (DOI: 10.3847/2041-8213/ab2a14). The research was funded by the National Aeronautics and Space Administration (Chandra award GO6-17094X and Hubble award HST-GO-14730.001-A), the National Science Foundation Graduate Research Fellowship Program (grant DGE-1656466), and the Simons Foundation.