Supermassive Black Holes Discovered in Close Tango

Astronomers have found what could be the closest known pair of supermassive black holes detected via direct imaging, orbiting each other only a light-year apart.

If you think your life is busy, imagine the life of Mrk 533. This active galaxy is the largest and brightest member of its group, and its tidal tails are testament to its busy social life — interactions with its companions are literally tearing the galaxy in all directions.

One of those interactions may have come a little too close. New observations reported in the September 18th Nature Astronomy reveal a double radio source at the center of Mrk 533, possibly the emission from a close pair of supermassive black holes separated by only a light-year in their orbits. The closest pair of directly imaged supermassive black holes known until now, discovered in the galaxy 0402+379, orbits 24 light-years apart.

Gravitational swipes from smaller galaxies are probably responsible for the gas that’s spiraling down to feed the putative pair of black hole beasts at the heart of Mrk 533. Even though we view the galaxy face on, observations show that we see the central black hole(s) equator-on, and a thick veil of dust and gas hides much of the emission. Preeti Kharb (National Centre for Radio Astrophysics, India) and colleagues used the Very Long Baseline Array (VLBA) to peer through this veil and into Mrk 533’s core. Probing various radio frequencies, the team found that the single source of radio emission resolved into two sources at 15 GHz.

One possibility is that the radio waves emanate from a pair of supermassive black holes with a combined mass of 40 million Suns, orbiting each other about a light-year apart. If so, then this is the closest pair of central black holes detected via direct imaging — a boon for observers and theorists alike.

Large galaxies are thought to grow by merging with other galaxies, and most large galaxies host a supermassive black hole. So it’s only natural that some large galaxies in the late stages of a merger would host not one but two black holes, bound together in an inward-spiraling orbit. But such pairs — especially close ones — have proven surprisingly difficult to find.

Without many examples to guide them, theorists have struggled to understand how two supermassive black holes swinging by each other at tremendous speed could lose enough angular momentum to merge. It could be that the black holes’ interactions with surrounding gas and dust helps them come together, but that remains to be seen in observations.

Including this one: This double radio core is still labeled as a candidate supermassive black hole binary. The VLBA observations only detect the double source in Mrk 533 at a single frequency, and without detections at other frequencies, it’s difficult to nail down the sources’ spectra and rule out alternatives for the radio emission. It’s possible, for example, that one of the radio sources is a black hole, but the other is simply the black hole’s jet rather than another black hole.

That said, there are some factors that favor the double black hole scenario. For example, Mrk 533's stubby radio-emitting jets, extending some 2,000 light-years, have an unusual Z shape. “This morphology is thought to result from the combined effects of the galaxy merger followed by the formation of the massive binary,” says study coauthor David Merritt (Rochester Institute of Technology).

“If the system is confirmed to be a SMBHB system with a separation of just 0.35 parsec [1 light-year], then this will be very interesting,” says Gregory Taylor (University of New Mexico). “This will improve our constraints on the occurrence rate of binary black holes in mergers, and tell us more about how binary black holes merge.”