Large galaxies like ours are hosts to Supermassive Black Holes (SMBHs.) They can be so massive that they resist comprehension, with some of them having billions of times more mass than the Sun. Ours, named Sagittarius A* (Sgr A*), is a little more modest at about four million solar masses.

Astrophysicists have studied Sgr A* to learn more about it, including its age. They say it formed about nine billion years ago.

SMBHs are the Universe’s most beguiling objects. They’re so massive that their gravitational pull can trap light. They’re surrounded by a rotating ring of material called an accretion disk that feeds material into the hole. When they’re actively feeding, they’re called active galactic nuclei (AGN.) The most luminous AGNs are called quasars, and they can outshine entire galaxies.

How can scientists determine the age of these confounding objects? How can they learn when our black hole, Sgr A*, formed? By gathering data, piecing it together, and running simulations.

This effort started in earnest in April of 2017 when the Event Horizon Telescope (EHT) observed the black hole at the center of galaxy M87. That was the first time we saw an image of a black hole, and it was followed up in 2022 when the EHT observed Sgr A*.

New research published in Nature Astronomy relied on EHT observations to ascertain Sgr A*’s age and origin. It’s titled “Evidence of a past merger of the Galactic Centre black hole.” The authors are Yihan Wang and Bing Zhang, both astrophysicists at the University of Nevada, Las Vegas.

Black holes grow in two ways. They accrete matter over time, and they merge. Astrophysicists believe that it takes a galaxy merger to form an SMBH, and Sgr A* is no different. It likely formed through a merger, though it also accretes material.

Sgr A* is unusual. It spins rapidly and is misaligned relative to the Milky Way. This is evidence of a past merger, according to Wang and Zhang, possibly with a long-gone satellite galaxy called Gaia-Enceladus.

“The Event Horizon Telescope (EHT) provided direct imaging of the SMBH Sgr A* at the Milky Way’s center, indicating it likely spins rapidly with its spin axis significantly misaligned relative to the Galactic plane’s angular momentum,” the authors write in their paper.

The pair of researchers used computer simulations to model what impact a merger would have on the Milky Way’s black hole. “Through investigating various SMBH growth models, here we show that the inferred spin properties of Sgr A* provide evidence of a past SMBH merger,” the authors write.

Their work shows that a 4:1 mass ratio merger with a highly inclined orbital configuration can explain what EHT observations of Sgr A* show. “Inspired by the merger between the Milky Way and Gaia-Enceladus, which has a 4:1 mass ratio as inferred from Gaia data, we have discovered that a 4:1 major merger of SMBH with a binary angular momentum inclination angle of 145-180 degrees with respect to the line of sight (LOS) can successfully replicate the measured spin properties of Sgr A*,” the authors explain in their work.

“This merger likely occurred around 9 billion years ago, following the Milky Way’s merger with the Gaia-Enceladus galaxy,” said Zhang, a distinguished professor of physics and astronomy at UNLV and the founding director of the Nevada Centre for Astrophysics. “This event not only provides evidence of the hierarchical black hole merger theory but also provides insights into the dynamic history of our galaxy.”

“This discovery paves the way for our understanding of how supermassive black holes grow and evolve,” said lead author Wang in a press release. “The misaligned high spin of Sgr A* indicates that it may have merged with another black hole, dramatically altering its amplitude and orientation of spin.”

“This merger event in our galaxy provides potential observational support for the theory of hierarchical BH mergers in the formation and growth of SMBHs,” the authors write in their conclusion.

When galaxies merge, so do their central black holes. While this has been largely theoretical, gravitational wave observatories are detecting an increasing number of black hole mergers. However, due to our observatories’ frequency range, they’ve only detected stellar mass black hole mergers. SMBH mergers would produce much lower gravitational wave frequencies that are beyond the range of detectors like LIGO/Virgo/KAGRA. The system’s detectors are too close together to detect the lower frequencies.

The authors also point to SMBH merger rates determined in other simulations like the Millenium Simulations, which suggests there could be hundreds or thousands each year in the observable Universe. “The inferred merger rate, consistent with theoretical predictions, suggests a promising detection rate of SMBH mergers for space-borne gravitational wave detectors expected to operate in the 2030s.”

There are plans to build facilities that can detect these lower SMBH merger frequencies. The ESA and NASA are planning a mission called LISA (Laser Interferometer Space Antenna) that can detect these waves. LISA will consist of three spacecraft working together as an interferometer. Each spacecraft would be 2.5 million km long.

SMBHs are some of the most puzzling objects in the Universe and are daunting to study. However, even in the absence of any gravitational wave evidence of SMBH mergers, this research helps set the stage for deepening our understanding of these mergers when they do occur.