Cosmological extremes: Researchers report on the largest jet pair known to date in the cosmos. The two jets that a supermassive black hole in the center of a galaxy sends into space have a total length of seven million parsecs – 23 million light years. This corresponds to the extent of 140 Milky Way galaxies lined up next to each other. The colossal structure dates back to a time when our universe was only half as old and much denser than it is today. Jets from black holes could therefore have influenced the formation of galaxies in the early universe more than previously thought, say the researchers.
They shoot concentrated streams of electrons, atomic nuclei and magnetic fields into their cosmic home and beyond: Some supermassive black holes in the center of galaxies make themselves noticeable through gigantic outflows – so-called jets. These fascinating structures have been the focus of astronomy for some time now. The details of their formation are still unclear, but they can basically be traced back to what is known as accretion – the feeding behavior of the gravitational giants: The jets are created by the extreme dynamic and magnetic effects that act on the disk of matter around active black holes.
It was already known that the streams extend very far into space and can therefore influence the flow of matter in the galactic and even intergalactic medium. Previous observations of black hole jets suggested that they do not exceed five million parsecs (one parsec is 3.26 light years). However, with its approximately seven million parsecs, the new record specimen now proves that the streams can escape destruction by cosmological instabilities even over even greater distances.
The largest jet system among many
Researchers led by Martijn Oei from Leiden University discovered it as part of a sky survey to search for the mysterious jets. The European radio telescope LOFAR (LOw Frequency ARray) was used. It is a network of many antenna units at different locations in Europe, whose signals are combined into a single signal. To search for jets, the radio data was evaluated using machine learning, among other things. This enabled the team to track down thousands of large jet systems of black holes.
“Giant jets were already known before we started the campaign, but we had no idea that so many would be found,” says co-author Martin Hardcastle from the University of Hertfordshire. Among the discoveries was the new record holder. Calculations showed that the structure has a total length of seven million parsecs, or about 23 million light years. It is formed by two jets that emanate in opposite directions from a supermassive black hole. “The Milky Way would be a small dot in these two gigantic structures. We are talking about a total of 140 Milky Way diameters,” says Oei.
How jets can extend so far beyond their host galaxies remains unclear. “My interpretation, however, is that unusually long-lived and stable accretion events are necessary around the supermassive black holes. This allows them to be active for so long that the jets point in the same direction the whole time. What we learn from the large number of giants found is that this appears to be the case relatively often,” says Hardcastle.
A look into cosmic history
The team named the discovered record jet structure “Porphyrion” – after a giant from Greek mythology. Using data from other telescopes, the researchers were finally able to locate the system: Porphyrion originates from a black hole that sits in the center of a galaxy that is about ten times more massive than our Milky Way and is located 7.5 billion light years from Earth. This distance is equivalent to a deep look into the past: we see Porphyrion in its state about 6.3 billion years after the Big Bang, the researchers explain. At that time, the universe had not expanded so far, which meant that the galaxies were closer together and the cosmic web was more densely structured than it is today.
The new findings now suggest that the giant jet systems may have had a greater influence on the formation of galaxies in the early universe than previously thought. “Galaxies and their central black holes are thought to evolve together, and a key aspect of this is that jets can spread huge amounts of energy that affect the growth of their host galaxies and other galaxies in their vicinity. So this discovery shows that their effects can reach much further than we thought,” says co-author George Djorgovski of the California Institute of Technology in Pasadena. Oei adds: “If distant jets can reach the scale of the cosmic web, then every place in the universe could have been affected by black hole activity at some point in cosmic time.”
The astronomer concludes: “We may only be seeing the tip of the iceberg. Our LOFAR survey only covered 15 percent of the sky. And most of these giant jets are probably difficult to detect, so we believe there are many more of these giants out there,” Oei says.
Source: California Institute of Technology, technical article: Nature, doi: 10.1038/s41586-024-07879-y