When galaxies merge, the supermassive black holes at their centers spiral around each other and eventually coalesce into a single black hole. How can we track down these massive mergers?

The Hunt for Massive Mergers

Modeling the emission from supermassive black hole binaries may help us to distinguish them from active galactic nuclei powered by single black holes. [NASA/JPL-Caltech]

Since the first detection of gravitational waves from a pair of stellar-mass black holes in 2015, gravitational waves have been a powerful tool to study merging black holes. However, detecting the extremely long-wavelength gravitational waves from merging supermassive black holes — with wavelengths of up to tens of light-years! — is beyond our current capabilities. What other methods can we use to detect supermassive black holes in the midst of merging?

One possibility is to track down the electromagnetic radiation produced by the hot plasma that surrounds the black holes as they draw closer. If we can detect this radiation, we can study supermassive black holes as they merge as well as potentially identify the small fraction of active galactic nuclei that are actually powered by black hole binaries rather than by a single black hole — a population that has never been definitively detected.

Simulated surface brightness of the accretion disks around merging black holes with a total mass of one million solar masses. Results are shown for three different wavelengths (left to right: 45, 12, 0.3 nanometers) and for spinning (top row) and non-spinning (bottom row) black holes. In all simulations, a gap opens between the circumbinary disk and the mini-disks — a feature that is not present in models of accretion disks around single black holes. Click to enlarge. [Gutiérrez et al. 2022]

Verging on Merging

A team led by Eduardo Gutiérrez (Argentine Institute of Radio Astronomy and Rochester Institute of Technology) used general relativistic magnetohydrodynamics simulations to model the electromagnetic radiation generated as two supermassive black holes approach a merger.

To predict the light emitted by the system, Gutiérrez and collaborators first modeled the motion of the superheated plasma surrounding the black holes. As the black holes circle around each other, the surrounding material forms a disk that envelops both black holes as well as mini-disks that circle each black hole. A dense region called the “lump” develops on the inner edge of the larger disk, periodically feeding material to the mini-disks.

The team then simulated the winding path that photons would take through the superheated plasma and warped spacetime to reach an observer on Earth. The resultant spectrum is mainly composed of emission from the disk around the binary, the mini-disks, and the streams of material that connect the larger disk to the mini-disks.

Seeing Double

Spectral energy distributions derived from simulations of accreting black holes. In the simulations, the mass of the single black hole is equal to the sum of the masses of the binary components. [Adapted from Gutiérrez et al. 2022]

Gutiérrez and coauthors found that the radiation from merging supermassive black holes should be detectable, and there are significant differences in the emission from merging black holes and a single black hole. Specifically, a binary system emits less energy than a single black hole, and its emission peaks at a lower frequency and decreases less sharply at frequencies above the peak. And unlike a single black hole, emission from black hole binaries should show semi-periodic behavior; because the lump that feeds material to the mini-disks has a slightly elliptical orbit, the accretion rate — and therefore the strength of the emission — increases when the lump passes closest to the mini-disks.

The authors predict that the signal from a black hole binary with a total mass of a billion solar masses would vary with periods of ~20 and ~150 days, while the emission from a million-solar-mass binary would vary on shorter timescales. Repeated X-ray observations should be able to detect this variation, determine whether the cause of the emission is one black hole or two, and give us the first-ever look at behemoth black holes moving toward a merger.

Citation

“Electromagnetic Signatures from Supermassive Binary Black Holes Approaching Merger,” Eduardo M. Gutiérrez et al 2022 ApJ 928 137. doi:10.3847/1538-4357/ac56de