🔭 A mapping of supermassive black holes near merger

Detecting supermassive black holes just before their merger would lead to significant advances in astrophysics. To achieve this, a new innovative approach combines the study of the gravitational wave background, these tiny vibrations of spacetime, with the observation of quasars.

Quasars are extremely luminous galactic nuclei, powered by supermassive black holes in full activity. When two of these giants approach each other to merge, they emit gravitational waves, creating a characteristic signal that scientists are learning to decipher.


By scrutinizing one hundred and fourteen active galactic nuclei, scientists have identified two particularly promising binary systems. Named Gondor and Rohan, in reference to The Lord of the Rings, these supermassive black holes offer a concrete example of the technique.

The presence of these gravitational waves around a quasar thus signals the probable existence of a pair of black holes on the verge of merging. This discovery allows for the consideration of creating a cosmic map cataloging these titanic events. Researchers estimate that even a small catalog of mergers could greatly improve our understanding of these phenomena.

The work, led by the NANOGrav project, paves the way for a systematic detection of supermassive black hole binaries. This advance also helps to better grasp the physics of merging galaxies and the fundamental nature of gravitational waves. The team published its results in The Astrophysical Journal Letters.

Gravitational Waves

Gravitational waves are perturbations of spacetime, predicted by Albert Einstein over a century ago. They propagate at the speed of light and are generated by violent events, such as the merger of black holes or neutron stars. Their direct detection, achieved for the first time in 2015, opened a new window on the Universe.

These waves are extremely tenuous: when passing through Earth, they alter distances in an infinitesimal manner, on the order of a billionth of a billionth of a meter. To capture them, scientists use very sensitive instruments, such as the LIGO and Virgo interferometers. These detectors measure the tiny variations in length between perpendicular arms, caused by the passage of a gravitational wave.

The gravitational wave background is a constant hum produced by the superposition of many distant sources. It is particularly useful for studying supermassive black hole binaries, whose mergers are too slow to be detected individually over short periods. By analyzing this background, astronomers can obtain statistical information about the population of these objects in the Universe.

This approach complements traditional observation by light, allowing the exploration of otherwise invisible phenomena. It has already confirmed key aspects of general relativity and promises many discoveries about the formation and evolution of cosmic structures.

Quasars and Supermassive Black Holes

Quasars are the extremely bright nuclei of distant galaxies, often called active galactic nuclei. Their prodigious luminosity comes from the accretion of matter around a central supermassive black hole. When gas and dust fall toward the black hole, they form a hot disk that radiates intensely before disappearing behind the event horizon.

These supermassive black holes typically weigh between millions and billions of times the mass of the Sun. They reside at the center of most large galaxies, including our own, the Milky Way. Their activity profoundly influences their host galaxy, regulating star formation and shaping galactic structure through the energy jets they can emit.

When two galaxies collide, their central black holes can end up forming a binary system. Spiraling around each other, they emit gravitational waves and, if they are active, appear as quasars. Thus, observing a quasar can indicate the presence of a supermassive black hole, and particular variations can betray an interacting pair.

The study of quasars thus allows tracing the distribution and behavior of supermassive black holes across the cosmos. By associating them with gravitational signals, scientists obtain a more complete picture of these objects.