Astronomers have discovered a group of faint “dying” radio galaxies that could change our understanding of what happens after supermassive black holes stop producing enormous jets of energetic particles.
The research, led by scientists at the University of Cape Town and the Inter-University Institute for Data Intensive Astronomy, suggests that the final stage of these galaxies may be much shorter and more active than previously thought.
The findings were published in the Monthly Notices of the Royal Astronomical Society.
At the center of many galaxies sits a supermassive black hole. When a black hole is actively consuming surrounding material, it can power enormous jets that shoot particles far into space at close to the speed of light.
These jets can create huge regions of radio-emitting material called lobes, sometimes extending far beyond the galaxy itself.
But the jets don’t last forever.
Eventually, the central black hole becomes less active and stops supplying the lobes with fresh energetic particles. The remaining lobes continue to glow for a while, but gradually fade as their particles lose energy. Astronomers call galaxies in this stage “remnant radio galaxies.”
Because these remnants can become extremely faint, they are difficult to find.
In the new study, researchers investigated 14 possible remnant radio galaxies using observations from several powerful radio telescopes, including South Africa’s MeerKAT. By examining radio waves across a wide range of frequencies, the scientists could estimate how the particles inside each galaxy had aged.
They confirmed that 12 of the 14 galaxies were genuine remnants. The other two turned out to still be active.
The confirmed remnants were surprisingly young.
Their estimated ages ranged from about 8 million to 42 million years, with a typical age of only around 12 million years. While that may sound ancient by human standards, it’s relatively brief in the life of a galaxy.
The findings suggest earlier surveys may have missed a large population of young remnants because they fade quickly and are difficult to detect.
The researchers also found that the galaxies were at very different stages of their decline. In some, the jets had only recently stopped. Others had spent most of their measured lifetime in the fading remnant stage.
Distance also appears to matter.
Several of the galaxies are extremely distant, meaning we see them as they existed when the universe was much younger. At these distances, energetic particles can lose energy more rapidly as they interact with radiation left over from the Big Bang, known as the cosmic microwave background.
The study found evidence that more distant remnants tend to appear younger, supporting the idea that they fade more quickly and therefore disappear from view sooner.
Detailed maps also showed that the giant radio lobes don’t simply stop changing when the black hole jets shut down. Particles continue moving through them, and their magnetic fields and surrounding environments can influence how they age.
The findings provide another piece of the puzzle surrounding the life cycles of supermassive black holes.
Future observations from the Square Kilometre Array could discover many more of these faint dying galaxies, allowing astronomers to better understand how often black holes switch their enormous jets on and off—and just how quickly their cosmic footprints disappear.
Source: KSR.
