Astronomers have discovered that one member of a pair of supermassive black holes is actually a pair itself, turning the system into the most distant black hole triplet yet detected and raising hopes for future discoveries.

All large galaxies host supermassive black holes. So we expect some galaxy mergers to produce systems that are home to pairs, or even trios, of supermassive black holes guzzling enormous amounts of matter, called active galactic nuclei (AGNs). But since there are only a handful of known galaxies with multi-AGN cores, astronomers have assumed that these systems are rare.

The recent discovery of a highly compact AGN trio challenges that assumption, astronomers suggested on June 25th in Nature online.

An artist's conception of an AGN. NASA / Dana Berry (SkyWorks Digital)
An artist's conception of an AGN.
NASA / Dana Berry (SkyWorks Digital)

A few years ago, astronomers detected the double-peaked spectrum characteristic of an AGN binary at the heart of SDSS J1502+1115, a quasar whose light traveled over four billion years to reach us. However, Roger Deane (University of Cape Town, South Africa) and colleagues have now resolved one of the two AGNs into two separate radio signals, indicating that the source is not one, but two accreting supermassive black holes. That means that, in all, J1502 has three supermassive black holes in its core. The AGN components of the inner binary are separated by 450 light-years and lie 24,000 light-years from the third AGN, making this system the tightest black hole triplet ever discovered.

The Power of VLBI

The inner AGN binary previously went undetected because the angular separation of the two sources is about 26 milliarcseconds, four times smaller than astronomers can usually resolve in the optical or near-infrared, explains Yue Shen (Carnegie Observatories), who co-discovered another AGN triplet in 2011. Deane’s team achieved the necessary resolution using very long baseline interferometry (VLBI). This technique combines observations from multiple radio telescopes in separate locations, such that the image resolution is equivalent to what you’d have if you used one telescope as large as the baseline separation between the telescopes — which can be thousands of miles. VLBI has proved highly successful with other black hole investigations, such as the growing Event Horizon Telescope, which aims to resolve the silhouette of the supermassive black hole at the center of the Milky Way Galaxy.

Map of the European VLBI Network.Consortium for Very Long Baseline Interferometry in Europe
Map of the European VLBI Network.
Consortium for Very Long Baseline Interferometry in Europe

Deane’s team used the European VLBI Network to observe several AGNs in detail 50 times finer than would be possible with the Hubble Space Telescope. Deane’s team only surveyed six galaxies before they discovered the compact AGN binary hiding in J1502, which indicates that these systems might be more common than previously thought.

That’s an exciting prospect, because astronomers have only resolved a few AGN binaries into their components in direct images. Of these, only three (not including J1502) have separations less than about 3,000 light-years, and they all reside in relatively nearby galaxies. But previous searches targeted some of the brightest, most massive AGNs (a billion solar masses or more), which are so rare that they are unlikely to have companions of comparable size.

Part of the problem is that astronomers can only find double or triple AGNs when all the black holes in the system are “on,” or voraciously eating material and glowing, explains Kevin Schawinski (ETH Zurich, Switzerland). “Astronomers have been searching for such systems for years, and at some level we know they ought to exist,” he says. Future VLBI surveys should shed more light on the prevalence of multi-AGN systems.

Fodder for Future Research

Artist's rendering of bent jets spewing off one AGN component of the inner binary.
Roger Deane/NASA Goddard

Even if they’re not using VLBI, astronomers might have more confidence in their binary searches thanks to the new study. Deane’s team also looked at archival observations from the Jansky Very Large Array and found that one component of J1502’s inner binary seems to be spewing out a pair of S-shaped jets. Astronomers have long predicted that the gravitational interference of a companion black hole could bend the jets of an AGN, but the JVLA observations, coupled with those from the VLBI network, provide the first direct evidence of this effect. In future AGN surveys, astronomers could therefore use warped jets to recognize multi-AGN systems, even if they can’t resolve the components.

Finding multi-AGN systems is a key part of studying galactic evolution. Astronomers think that supermassive black holes in AGN phase can influence how their host galaxies evolve, but the exact nature of the AGN-host galaxy relationship remains a topic of debate. Knowing how supermassive black holes interact with each other is crucial to understanding their effects on galaxies.

Multi-AGN systems are also prime targets for research on gravitational waves – ripples in spacetime caused by accelerating, massive objects. Astronomers know very little about black hole systems compact enough to emit detectable gravitational waves (and they haven’t directly detected any of these waves yet), but the authors suggest that their discovery heralds the discovery of many more tight multi-AGN systems using VLBI, which could bode well for targeted gravitational wave studies.

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