At the center of the galaxy Markarian 501, there appears to be not just one supermassive black hole, but two. Radio observations over several years suggest that the duo could merge in as short as 100 years.
The artistic rendering shows the center of the galaxy Markarian 501, from which two powerful jets emanate. The supermassive black hole at the centre, whose existence was already known, partially bends the light from the jet behind it into a so-called Einstein ring. This curved jet most likely originates from a second, unobserved black hole. The radio observations are visible as contours in the background. Emma Kun / HUN-REN Konkoly Observatory / Made with the support of AI
Emma Kun / HUN-REN Konkoly Observatory / Made with the support of AI
To the point
An international research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy (MPIfR) has imaged two large particle streams (jets) in the core of a galaxy.
It is the first image of its kind created using an international network of radio telescopes, and it provides direct evidence of a pair of extremely massive black holes orbiting each other very closely, each acting as a "motor" for jets of matter.
The pair is believed to be in the final phase before merging. Until now, it was unclear whether this phenomenon could exist and whether it could be observed.
Current findings suggest that there is a supermassive black hole at the centre 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 centres 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. An international team led by Silke Britzen from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn found direct evidence of such a pair at the heart of Mrk 501. Their work has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society, and will appear in an upcoming issue.
The black hole at the centre of Mrk 501 ejects a powerful jet of particles travelling at nearly the speed of light into space. For the study, the team analysed 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 centre 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 Silke Britzen.
Close dance of black holes
The first jet points towards 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 Silke 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. One explanation consistent with the interpretation of a binary black hole system would be that the system was perfectly aligned with us.. Gravitational lensing by the known black hole in front then shaped the light of the second jet behind it.
By analysing 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 favoured 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.
The graphical depiction shows the central region of the galaxy Mrk 501 at a frequency of 43 gigahertz on three different days. The contours indicate the intensity of the emission, while the grey circles mark bright regions within the jet, identified through model calculations. One can track the movement of the jets by following the movement of these regions. The previously known jet (Jet 1, orange guide line) pointing towards Earth is clearly visible. The newly discovered second jet (Jet 2, blue) changed its appearance within a few weeks. Both particle streams originate close to each other in the core of the galaxy. The position of the black hole (BH) associated with Jet 1 is marked with an arrow.
© S. Britzen
Additional Information
The following scientists affiliated to the MPIfR are co-authors of this publication: Silke Britzen, Frédéric Jaron und Nicholas Roy McDonald.
