Astronomers have discovered a supermassive black hole not sitting in its customary seat at the center of its galaxy. Gravitational waves from a recent merger may have ejected the black hole.

Merged supermassive black hole, 3C 186
The bright, star-like quasar 3C 186 and its former host galaxy, the faint, extended object behind it, sit at the center of a galaxy cluster. The quasar was probably ejected following a long-ago galaxy merger.
NASA / ESA / M. Chiaberge (STScI / ESA)

A billion years ago in a universe far away, two galaxies collided. Stars swirled into new orbits and gas clouds slammed into each other. New stars were born and gas funneled into the two galaxies’ centers, feeding the two supermassive black holes lurking there.

Then, about 5 million years ago, these black holes fused into one. As they whirled around each other, they radiated energy in the form of gravitational waves —a signal that a space-based detector such as LISA might one day detect — and those waves gave the final product a real kick in the pants. The merged supermassive black hole, an object with a mass roughly 3 billions times the Sun's, hastened away from the new galaxy center at millions of miles per hour.

This is the picture that Marco Chiaberge (Space Telescope Science Institute and John Hopkins University) and colleagues have painted to explain the unique observations of the quasar known as 3C 186. The results were presented in the April 2017 Astronomy & Astrophysics.

The Evidence

The quasar itself is a brilliant beacon of light emitted by gas that’s plummeting down into the central black hole. Now, new Hubble Space Telescope images for the first time reveal the galaxy that hosts this point of light. And, surprisingly, the images show that the quasar is offset from the galaxy center by about 1.3 arcsecond.

That’s a tiny angle on the sky — equivalent to the width of a human hair held 10 meters away. But in a galaxy so far away that its light must travel 8 billion years to get here, 1.3 arcseconds translates to a distance of 35,000 light-years. For comparison, the Sun sits only 26,000 light-years from our galaxy’s center.

The team also carefully analyzed the quasar’s visible-light spectrum, revealing the chemical signature of gas speeding out of the host galaxy. (Earlier, automated analyses of the spectrum had missed the signature.) The signature is that of a black hole and its surrounding gas buffet flying away from the galaxy center at 2,140 kilometers per second (4.8 million mph). The black hole will shine brightly as long as it holds onto its gas disk, probably another 100 million years or so according to the researchers’ calculations.

Together, the quasar’s incredible speed and its offset from the galaxy’s center point to one likely conclusion: this black hole is the product of two unequal black holes, whose lopsided merger emitted asymmetric gravitational waves and ultimately kicked the black hole out of its central galactic home. This isn’t the first such candidate discovered, but it’s the first to have two signatures of gravitational wave recoil — previous candidates have shown spatial offset or incredible speed but not both.

Merging supermassive black holes
As two galaxies interact and merge, their central black holes are gravitationally attracted to each other (first panel). When the two black holes become close enough together, they send out gravitational waves, shrinking their orbit over time (panel 2). Eventually, they merge (panel 3). Since the black holes were initially different masses, and their gravitational waves correspondingly lopsided, the merger will give the new, more massive black hole a firm kick out of the galaxy.
NASA / ESA / Hubble / A. Feild / STScI

“3C 186 is an exciting candidate for a gravitational wave recoiling supermassive black hole, which have proven very difficult to find,” says Julie Comerford (University of Colorado, Boulder), who was not involved in the study.

But, as both Chiaberge’s team and Comerford point out, this candidate isn’t a shoo-in just yet. There’s a chance, for example, that the quasar lies in a second, less luminous galaxy that’s in the background of the galaxy that Hubble imaged. So Chiaberge and colleagues are already planning follow-up observations with Hubble in combination with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to rule out alternative possibilities.

The Final Parsec Problem

An artist's concept shows two merging black holes. NASA
An artist's concept shows two merging black holes. They emit gravitational waves as they whirl around in their orbits, and that gradually brings them closer together. But here's the tricky part: if two black holes are too far apart, they won't radiate enough energy — something must bring them close enough to begin with for gravitational waves to do the work. 

Astronomers are pretty sure black holes merge on a regular basis — otherwise, it’s awfully hard to explain how we see billion-solar-mass black holes just a billion years after the Big Bang. Astronomers also know that galaxy collisions can deliver two supermassive black holes pretty close to each other. And they have calculated that gravitational waves can radiate away enough energy to bring about the final union.

But galaxies don’t deliver their central black holes quite close enough together for gravitational waves to do the rest of the work. There must be something that gets the black holes the light-year or three closer to each other to complete the merger process. This conundrum is known as the Final Parsec Problem. If 3C 186 really is the final product of two supermassive black holes, it could serve as the perfect testbed for how such fusions happen.

“The observations provide strong evidence that supermassive black holes can actually merge,” says study coauthor Stefano Bianchi (Roma Tre University, Italy).

Now astronomers just have to figure out how.


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