Researchers spot largest black hole jets ever discovered

The jets are 140 times larger than the Milky Way.

The supermassive black holes that sit at the center of galaxies aren't just decorative. The intense radiation they emit when feeding helps drive away gas and dust that would otherwise form stars, providing feedback that limits the growth of the galaxy. But their influence may extend beyond the galaxy they inhabit. Many black holes produce jets and, in the case of supermassive versions, these jets can eject material entirely out of the galaxy.

Now, researchers are getting a clearer picture of just how far outside of the galaxy their influence can reach. A new study describes the largest-ever jets observed, extending across a total distance of 23 million light-years (seven megaparsecs). At those distances, the jets could easily send material into other galaxies and across the cosmic web of dark matter that structures the Universe.

Extreme jets

Jets are formed in the complex environment near a black hole. The intense heating of infalling material ionizes and heats it, creating electromagnetic fields that act as a natural particle accelerator. This creates jets of particles that travel at a substantial fraction of the speed of light. These will ultimately slam into nearby material, creating shockwaves that heat and accelerate that, too. Over time, this leads to large-scale, coordinated outflows of material, with the scale of the jet being proportional to a combination of the size of the black hole and the amount of material it is feeding on.

Typically, black holes form a jet at each of their poles, resulting in paired outflows traveling in opposite directions. We've seen plenty of examples of these at various scales, launched from stellar-mass black holes all the way up to supermassive ones, which can form quasars, the brightest objects in the Universe.

The new jet discovery came as a result of an organized search for large jets, done at radio wavelength at an observatory called LOFAR (low-frequency array) that covers portions of the Northern Hemisphere's sky. The data obtained with that telescope has been scanned by a combination of machine learning and citizen scientist volunteers. This program has identified over 11,000 jets that cover megaparsec distances (each parsec is a bit over 3 light-years). The paper that was released on Wednesday describes the largest of these, which has been named Porphyrion after a giant from Greek mythology.

Initial follow-up observations involved finding the galaxy that produced it. There were two objects roughly in the right place, but one of those had lobes that were extended along the axis of the jets, indicating that it was the most probable source. The galaxy is roughly 10 times more massive than the Milky Way, and spectroscopy indicates that we're looking at it as it existed roughly 6 billion years after the Big Bang, or a bit over halfway back from the present day.

Based on that distance, the researchers estimate that Porphyrion is visually 6.4 megaparsecs long. That will only reflect its actual length if we're looking at it from a perpendicular angle. Chances are good that it's actually somewhat tilted from our perspective. The researchers estimate that the actual physical distance is a bit over 7 megaparsecs, but it's not an exact measurement.

The researchers do modeling to estimate that the material in the jets is moving at roughly 0.012 times the speed of light, meaning it would take over half a billion years to reach its present size, suggesting that the black hole that launched them has had a long period of active feeding. Overall, they estimate that the material in the jet represents an export of roughly 10⁵⁵ Joules into the intergalactic medium, energies that are normally seen in collisions of entire galaxy clusters.

Cosmological implications

One of the remarkable things about the jets is how straight they are, suggesting the material in them hasn't felt the influence of any other nearby galaxies, or run into any irregular patches of intergalactic material. They conclude that this means its host galaxy is in a filament of the dark matter web that permeates the Universe and provides the gravitational pull needed to gather enough material to form galaxies. Given its size, that would mean Porphyrion has likely blasted material back out of the filament entirely, and the jets are largely sailing through a material-poor void.

As mentioned above, there are a lot of big jets out there. However, the researchers also note that (as should be obvious from the image above) Porphyrion is barely detectable above the noise level inherent in today's radio astronomy equipment. If it were just a bit less energetic or a bit deeper back into the Universe's history, we wouldn't have been able to detect it.

Add in the fact that this project only searched about 15 percent of the total nighttime sky, and it's clear that we're probably missing a lot of similarly sized jets that existed early in the Universe's history. The researchers crunch the numbers and derive an estimate that every dark matter filament in the cosmic web probably has had a pair of jets that are similar in scale.

What does this mean for the Universe as a whole? The fact that these jets are spewing material back out of the dark matter filaments means that, at some level, they're providing feedback on the growth of galaxies in general. But the scale of these things means material from one galaxy may find its way into an entirely different galaxy. And, given that filaments were closer together early in the Universe's history, it's possible that some material moved between filaments, carried along on black hole jets.

It's unclear at this point whether these might be influential factors in the Universe's evolution. But the work suggests that it's probably worth throwing a few of these jet systems into models and looking at how large their influence might be.

Nature, 2024. DOI: 10.1038/s41586-024-07879-y  (About DOIs).