Astronomers are investigating a new technique for finding close pairs of supermassive black holes, and they might have found one in the nearest quasar.
Our universe should be teeming with waltzing pairs of supermassive black holes. Yet they’re incredibly difficult to find. A research team has proposed a new way to find these dancing duos, but whether the method actually works is up for debate.
At the center of every large galaxy (and many not-so-large ones too) sits a supermassive black hole millions or billions of times the mass of the Sun. And many of these galaxies have encountered and merged with at least one other galaxy. So it stands to reason that as galaxies meet and collide, their central black holes should meet, whirling around each other for at least a few hundred million years before they coalesce.
The trouble lies in finding these duos. To start, no current telescope can make out the finer details around a supermassive black hole in another galaxy. (Even resolving the dark beast lurking in our own galaxy proves difficult.) That means we can’t see directly the clouds of gas that orbit and feed the black hole or the wind that might arise from its accretion disk. Such intricate details are inferred instead from spectroscopic observations.
Telescopes can make out pairs of supermassive black holes as long as at least one of them is feeding. The gas funneling into the spacetime ditch heats up as it falls in, emitting brilliant radiation from optical to ultraviolet and beyond.
But even cutting-edge telescopes still have a hard time resolving black hole pairs if they’re too close together — one of the closest confirmed pairs that astronomers have spotted so far is still separated by 24 light-years. And it’s within 3 light-years that the black hole dance gets really interesting: there, the waltz might transform into more of a gravitational tango, providing astronomers with the perfect test bed for the finer points of general relativity.
Some astronomers have turned to spectroscopy to identify closer pairs. When a black hole feeds on gas, it produces many spectral lines. If two black holes are feeding on gas while simultaneously whipping around each other, they will each produce their own set of spectral lines. The Doppler effect shifts each set of lines according to the black holes’ motions: one set of spectral lines will shift redward as that black hole swings away from Earth in its orbit, while the other set shifts blueward as that black hole’s orbit brings it toward Earth.
But nature has ways of making double spectral lines without the presence of a second black hole. While some cases look quite convincing, they’ve proven difficult to confirm.
Now one team of astronomers, led by Chang-Sho Yuan (Chinese Academy of Science, Beijing), is taking a new approach (one that doesn’t involve time-consuming spectra). They put a ‘til-now purely theoretical idea into play in a paper published in Astrophysical Journal (arXiv preprint available here).
A Hole in the Data
Theory says that, in a certain stage of their spiraling dance, the mutual orbit of two black holes will carve out a hole in the center of the accretion disk that surrounds and feeds them both. Since the center of the disk is the hottest part and produces the shortest-wavelength emission, if the center of the disk disappears, so does the hot gas and the short-wavelength emission it produces. In other words, a pair of feeding supermassive black holes will emit plenty of visible light, but very little ultraviolet.
Yan and colleagues applied this idea to images of the nearest quasar, Mrk 231, whose light takes 575 million light-years to reach Earth. Astronomers have long known of this quasar’s strange light distribution — it produces far less ultraviolet radiation than it should. That’s been difficult to explain in terms of obscuring dust. (Dust tends to scatter away ultraviolet light, while letting optical light pass through, but not in quite the right way to explain this quasar’s spectrum.)
Instead, Yan’s team shows that Mrk 231’s output matches what’s expected from the gap scenario. To do that, one black hole would have to outweigh the other, with respective masses of 150 million Suns and 4.5 million Suns, and they would orbit closely, separated on average by just 590 times the Earth-Sun distance and completing an orbit every 1.2 years.
Mrk 231 does show signs of having been through a recent merger, so it could have a binary black hole. But not everyone’s convinced. Kelly Denney (Ohio State University) points out that Mrk 231 is also known to have all kinds of gas flowing out from the central region, as well as a burst of central star formation. These facts alone could potentially explain the quasar’s spectrum without a second supermassive black hole.
Unfortunately, if the binary is there, the orbit is too close to resolve with today’s telescopes, making it difficult to confirm. However, the authors suggest that future gravitational wave telescopes could listen for Mrk 231’s signal.
Meanwhile, the hunt for waltzing supermassive black holes continues. In today’s Nature, another team takes yet another approach, this time studying how a quasar’s light changes over time. And they find some pretty promising things . . . Take a look in our post on “New Evidence for Black Hole Binary.”
Why do we know there's a black hole in every large galaxy? How have black holes affected the evolution of the universe? Could we one day image a black hole's silhouette? Find out in our FREE Black Holes ebook.