A jet of superheated matter shooting from a galaxy more than a billion light-years away has been knocked off course. Astronomers believe dark matter is responsible, and the finding could change how scientists track the origins of some of the most mysterious particles in the universe.
The galaxy at the center of the discovery is a blazar called PKS 2233-148. Blazars are a type of active galactic nucleus that fire jets of ionized matter from each pole at nearly the speed of light. What makes PKS 2233-148 especially useful to researchers is that one of its jets points almost directly at Earth, giving astronomers an unusually clear view. According to a report published in Monthly Notices of the Royal Astronomical Society, a team led by Silke Britzen at the Max Planck Institute for Radio Astronomy in Germany found that the jet had been suddenly displaced from its expected path. The most likely explanation, the researchers say, is gravitational lensing by an unseen concentration of dark matter sitting somewhere between the blazar and Earth.
Gravitational lensing occurs when mass bends the path of light passing nearby. It is a well-documented effect, but lensing caused specifically by dark matter, which emits no light and cannot be directly observed, is far harder to confirm. The displacement of the PKS 2233-148 jet, the team argues, fits that explanation.
The blazar has attracted scientific interest in part because of its connection to cosmic neutrinos. These are subatomic particles with almost no mass and no electrical charge that travel across the universe at nearly the speed of light. They are notoriously difficult to detect. The IceCube Neutrino Observatory, buried deep in Antarctic ice, uses thousands of sensors spread across a full cubic kilometer to catch the faint flashes of light that appear when a cosmic neutrino strikes an ice molecule. By tracing those flashes, scientists can estimate where in the universe the neutrino originated.
PKS 2233-148 is considered a likely source of cosmic neutrinos because blazar jets are powerful enough to accelerate particles to the extreme energies needed to produce them. "These large-scale jets are cosmic accelerators, and might be generating neutrinos," Britzen said. "We study them to search for any peculiarities which might help us to gain a better understanding of neutrino emission."
To study the jet in detail, Britzen's team combined data from several observatories. Radio observations came from the Very Long Baseline Array, a network of dish antennas spread across the United States. Gamma-ray data came from the Fermi-LAT space telescope. X-ray measurements came from the Swift-XRT observatory. Together, those sources allowed the team to map the jet's motion and track changes in its behavior over time.
The results revealed more than just the displacement. The team also detected unexpected patterns in the gamma-ray data coming from the blazar. "We are very happy to have discovered as-yet-undetected phenomena in the jet, as well as in the gamma-ray light curve," Britzen said. That gamma-ray signal showed what the researchers described as a very fast flash sitting on top of a more regular background pattern, a behavior that fits with what would be expected if the jet's light were being bent and briefly focused by a gravitational lens.
The implications reach beyond this single blazar. If dark matter can gravitationally lens a jet like this one, it means that some of the signals astronomers use to trace cosmic neutrinos back to their sources could be subtly distorted without anyone realizing it. Understanding that distortion is a necessary step toward building a clearer picture of where these particles come from and how they are produced.
The team's findings are published and available for review by other researchers in the field.
