Using small black holes to find big black holes

Scientists have developed a new method for detecting supermassive binary black holes, pairs of the largest kind of black holes in the Universe and located at the very centre of galaxies.

The new technique will require a deci-Hz gravitational-wave detector and would enable astronomers to study supermassive black hole binaries, which might otherwise remain inaccessible.

Their approach analyses the gravitational waves – ripples in spacetime – emitted from small neighbouring black holes, which are themselves remnants of stars.

This exposes a “loophole” in targeting pairs of supermassive black holes orbiting each other, so-called binaries, which are too low in frequency for existing detectors, according to the team from Cardiff University, the Max Planck Institute for Astrophysics, the University of Zurich, the Niels Bohr Institute, and the California Institute of Technology.

Their technique instead takes advantage of the modulation in signals from smaller black holes located in the same galaxy, to indirectly reveal the presence of their supermassive neighbours.

The method, presented in Nature Astronomy, could help identify previously hidden black hole binaries with masses ranging from 10 million to 100 million times that of the Sun, even at vast distances, according to the team.

Lead author Dr Jakob Stegmann, a postdoctoral research fellow at the Max Planck Institute for Astrophysics, said “Our idea basically works like listening to a radio channel. We suggest using the signal from pairs of small black holes in a similar way to how radio waves carry a signal.”

The method leverages the subtle changes caused in the gravitational waves emitted by a pair of nearby small stellar-mass black holes.

“The small black hole binary effectively works as a beacon revealing the existence of the bigger black holes. The idea is to use high frequencies that are easy to detect to probe lower frequencies that neither currently existing gravitational-wave detectors nor instruments planned for the future will be sensitive to,” added Dr Stegmann who started this work during his doctoral study at Cardiff University.

The origin of supermassive black holes is still one of the biggest mysteries in astronomy.

They may have always been massive and formed when the Universe was still very young. Or they may have grown over time by gathering matter and other black holes in their wake.

Dr Fabio Antonini, one of the study’s co-authors and a Senior Lecturer at Cardiff University’s School of Physics and Astronomy, added: “Supermassive black holes are linked to their host galaxies in fundamental ways.”

While indirect evidence for gravitational waves from merging supermassive black holes already exists, it comes from the collective signal of many distant binaries that effectively create background noise.

Planned future detectors, such as the space-based European Space Agency-led mission Laser Interferometer Space Antenna (LISA), will go some way in remedying this, but detecting the most massive black hole pairs will still remain extremely challenging. 

Co-author Professor Lucio Mayer from the University of Zurich’s Department of Astrophysics, said: “As the path for the LISA is now set, after adoption by the ESA last January, the community needs to evaluate the best strategy for the following generation of gravitational detectors, above all in which frequency range to focus.”

The paper, ‘Imprints of massive black hole binaries on neighbouring decihertz gravitational-wave sources’, is published in Nature Astronomy.