A galaxy 500 million light-years away may host two supermassive black holes that will merge within about a century.

Analysis of 23 years of radio observations of Markarian 501 indicates two distinct jets and a 121-day cycle, which may indicate a pair of supermassive black holes close to merging, although at this stage it is still only a candidate.

Astronomers have repeatedly seen galaxies collide and merge, but one of the most dramatic stages in the process has remained largely unseen: the moment when two supermassive black holes, each once at the center of a different galaxy, finally end up together in the core of a single galaxy and begin to orbit each other in a tight circle. Now, a team of researchers says they have found a particularly compelling sign of such a scenario in the galaxy Markarian 501, a particularly active galaxy about 500 million light-years away.

According to the report, the heart of Markarian 501 emits two beams of radiation in different directions, a phenomenon that may indicate the existence of two separate jets, and therefore perhaps also the existence of two separate black holes at the heart of the same galaxy. If this interpretation is correct, this is a pair of extremely massive black holes: each of them has an estimated mass of about a billion solar masses. The researchers estimate that the two bodies complete one orbit around each other every 121 days, and at this rate they could merge within about a century. Such a merger, if it does occur, is expected to produce an extremely intense burst of gravitational waves.

The lead researcher, Silka Britzen of the Max Planck Institute for Radio Astronomy, believes that this is one of the most promising opportunities seen so far for identifying a pair of supermassive black holes in the final stages of their approach. However, the report itself also emphasizes that great caution is needed. Previous candidates for such pairs have been proposed in the past, but have not been confirmed. Astrophysicist Zoltan Heiman of Columbia University, who is not part of the team, said that the picture is still “very complicated,” and that at this point it is better to see the system as just a candidate.

Supermassive black holes are a natural product of galaxy mergers.

To understand why this is such an important discovery, it is important to remember that supermassive black holes are supposed to be a natural product of galaxy mergers. When two galaxies merge, the black holes at their centers do not immediately collide. First, they pass each other, enter into mutual orbits, and over hundreds of millions of years gradually lose energy through interactions with gas, stars, and finally through the emission of gravitational waves. The problem is that the final stage, when the two black holes are very close to each other, is very difficult to observe directly. Telescopes are usually unable to separate two objects that close together, so astronomers are forced to look for indirect clues, such as periodicities in light or the unusual behavior of radio jets.

Markarian 501 is a particularly suitable target for such a search because it is a blazar, an active galaxy whose core is rapidly swallowing material while simultaneously emitting a jet of particles and powerful radio signals that are generally directed toward Earth. Previously discovered anomalies in its radio signal have already raised suspicions that a pair of black holes may be hiding there. Britzan and her colleagues went back to the data and examined 23 years of observations collected using the Very Long Baseline Array radio telescope in the United States. At first, they did not see any clear motion, but when they switched to a higher frequency, which allows them to monitor the region closer to the black hole, they suddenly discovered a different signature, in a direction that does not match the familiar jet.

Second jet

The researchers interpreted the new signal as a second jet, possibly originating from a second black hole. In addition, they detected periodic brightness changes that repeat every 121 days, a figure that supports the interpretation of a reciprocal orbit. Here too, a healthy scientific reservation is heard: Daniel D'Orozio of the Space Telescope Science Institute noted that while it is intriguing to see that the second jet behaves differently from the first, which may indicate a different source, he cautioned that finding a system so close to a merger would be almost too good to be true. According to him, if the identification is confirmed, it will also be necessary to ask why many more systems of this type have not been found.

The road to confirmation or disproof may be relatively short in astronomical terms. If this is indeed a pair of black holes so close to merging, it may be possible to measure a real shortening of their orbital period within the next decade. In addition, according to the researchers, the system should already be emitting weak gravitational waves, which may be detected by monitoring the tiny changes in the regular pulses of pulsars. If this happens, it would be one of the most impressive confirmations yet of a close merger of supermassive black holes, and also a rare window into a final and particularly elusive stage in the evolution of galaxies.

for the scientific article

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