Other Apples

First close pair of supermassive black holes detected

Supermassive black holes at the centers of galaxies are one of the most active fields of research in astronomy. In order to accumulate their enormous masses, they must merge with each other. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn has found direct evidence of two supermassive black holes in the galaxy Markarian 501, which orbit each other very closely. This could be the first time that a pair has been detected that is about to merge. This provides a unique opportunity to better understand a central process in galaxy evolution.

The findings suggest that there is a supermassive black hole at the center of almost every large galaxy, with a mass millions or even billions of times greater than that of our sun. It is still unclear exactly how they can reach such enormous masses. Collecting (accreting) gas from the surrounding area alone would take too long, so it is likely that they have to merge with other massive black holes. Galaxy collisions have been observed throughout our universe. It is thus very likely that the supermassive black holes at the centers of these colliding galaxies also merge, first orbiting each other ever closer and ultimately coalescing into one.

Telltale particle beam

However, theoretical models cannot yet accurately describe this final phase. Complicating matters further, no close pair of massive black holes has yet been reliably detected, despite collisions between galaxies being commonplace on cosmic timescales. The recent study of the galaxy Markarian 501 (Mrk 501) in the constellation Hercules has changed that. The work has been published in the Monthly Notices of the Royal Astronomical Society.

The black hole at the center of Mrk 501 ejects a powerful jet of particles traveling at nearly the speed of light into space. For the study, the team analyzed high-resolution observations of the region. These cover various radio frequencies and were collected on dozens of days over a period of approximately 23 years. This long-term data reveals not only a single jet, but a second one as well. It is the first direct image of such a system at the center of a galaxy, and a clear indication of the existence of a second supermassive black hole. "We searched for it for so long, and then it came as a complete surprise that we could not only see a second jet, but even track its movement," reports Britzen.

Close dance of black holes

The first jet points toward Earth, which is why it appears particularly bright to us and has been known for a long time. The second jet is oriented differently and was therefore more difficult to detect. Over a period of just a few weeks, the astronomers observed significant changes: The second jet starts behind the larger black hole and moves counterclockwise around it. This process repeats itself.

"Evaluating the data felt like being on a ship. The entire jet system is in motion. A system of two black holes can explain this: The orbital plane sways," explains Britzen. On one observation day in June 2022, the radiation emitted by the system reached us on such a crooked path that it appeared ring-shaped—a so-called Einstein ring. The most likely explanation is that the system was perfectly aligned toward us. Gravitational lensing by the known black hole in front then shaped the light of the second jet behind it.

By analyzing the progression over time and recurring patterns in the brightness of the jets, the researchers were able to deduce that the two black holes orbit each other with a period of approximately 121 days. They are about 250 to 540 times farther apart than the distance between Earth and the sun—tiny for such extreme objects with masses of between 100 million and a billion times that of the sun. Depending on their actual masses, the distance between them could decrease so rapidly that they could merge in as short as 100 years.

Countdown to the finale

Due to the great distance between Mrk 501 and Earth, even the most advanced observation methods cannot image the two black holes as separate objects. Not even the Event Horizon Telescope (EHT), which provided us with the first images of black holes in 2019 and 2022, is powerful enough. The increasingly shrinking orbit of the pair in Mrk 501 will therefore not be directly observable. Nevertheless, scientists expect clear evidence of the ever-decreasing separation between the two black holes: The system should emit gravitational waves at very low frequencies, which could be detected using pulsar timing arrays (PTAs).

Supermassive black hole binaries (SMBHBs) are already the favored explanation for the observed gravitational wave background, for which evidence was found in 2023 by the European Pulsar Timing Array and others. Mrk 501 is now a prime candidate for attributing gravitational wave emission measured with PTAs to a specific supermassive black hole binary. "If gravitational waves are detected, we may even see their frequency steadily rise as the two giants spiral toward collision, offering a rare chance to watch a supermassive black hole merger unfold," notes co-author Héctor Olivares.