In 1974, astronomers Bruce Balick and Robert L. Brown discovered a powerful radio source at the center of the Milky Way galaxy. The source, Sagittarius A*, was subsequently revealed to be a supermassive black hole (SMBH) with a mass of over 4 million Suns. Since then, astronomers have determined that SMBHs reside at the center of all galaxies with highly active central regions known as active galactic nuclei (AGNs) or “quasars.” Despite all we’ve learned, the origin of these massive black holes remains one of the biggest mysteries in astronomy.
The most popular theories are that they may have formed when the Universe was still very young or have grown over time by consuming the matter around them (accretion) and through mergers with other black holes. In recent years, research has shown that when mergers between such massive objects occur, Gravitational Waves (GWs) are released. In a recent study, an international team of astrophysicists proposed a novel method for detecting pairs of SMBHs: analyzing gravitational waves generated by binaries of nearby small stellar black holes.
The study was led by Jakob Stegmann, a Research Fellow at the Max Planck Institute for Astrophysics (MPA) and the Gravity Exploration Institute at Cardiff University. He was joined by researchers from the Niels Bohr Institute, the Center for Theoretical Astrophysics and Cosmology at the University of Zurich (CTAC-UTZ), and the California Institute of Technology (Caltech). The study that describes the team’s findings, “Imprints of massive black-hole binaries on neighboring decihertz gravitational-wave sources,” recently appeared in Nature Astronomy.
First detected in 2015 by scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO), Gravitational Waves (GWs) are ripples in spacetime caused by the merger of massive objects like white dwarf stars and black holes. While multiple signals involving binary pairs of merging black holes have been detected, no GW events involving SMBHs have been detected because current Earth-based detectors are not sensitive to the very low frequency these events emit. Much like the issues facing ground-based observatories, scientists hope to remedy the situation by developing space-based instruments.
This includes the proposed Laser Interferometer Space Antenna (LISA), an ESA-led mission that is expected to launch sometime in 2035. Unfortunately, detecting mergers between the largest black holes in the Universe will still be impossible. However, Stegmann and his colleagues propose that binary SMBHs can be detected by analyzing the gravitational waves generated by smaller black hole binaries. Their proposed method leverages the subtle changes SMBHs cause to the GWs emitted by a pair of nearby smaller black holes.
In this respect, small black hole binaries work as a beacon, revealing the existence of larger pairs of merging black holes. As Stegmann explained in a recent UHZ press release:
“Our idea basically works like listening to a radio channel. We propose to use the signal from pairs of small black holes similar to how radio waves carry the signal. The supermassive black holes are the music that is encoded in the frequency modulation (FM) of the detected signal. The novel aspect of this idea is to utilize high frequencies that are easy to detect to probe lower frequencies that we are not sensitive to yet.”
However, the evidence that this proposed method offers would be indirect, coming from the background noise collectively generated by many distant binaries. Furthermore, it will require a deci-Hz gravitational-wave detector, which is far more sensitive than current instruments. For comparison, the LIGO detector measures GWs in the 7.0 kHz to 30 Hz range, whereas the Virgo Observatory can detect waves in the 10 Hz to 10000Hz range. By detecting the tiny modulations in signals from small black hole binaries, scientists could identify merging SMBHs ranging from 10 to 100 million Solar masses, even at vast distances.
As Lucio Mayer, a black hole theorist at the University of Zurich and a co-author of the study, added:
“As the path for the Laser Interferometer Space Antenna (LISA) is now set, after adoption by ESA last January, the community needs to evaluate the best strategy for the following generation of gravitational wave detectors, in particular which frequency range they should target – studies like this bring a strong motivation to prioritize a deci-Hz detector design.”
Further Reading: UZH, Nature Astronomy