Researchers from the University of Cape Town (UCT) and the Inter-University Institute for Data Intensive Astronomy (IDIA) led the study, which investigated 14 possible remnant radio galaxies in the XMM-Newton Large-Scale Structure (XMM-LSS) field.

Remnant radio galaxies are systems in which the jets produced by the central active galactic nucleus (AGN) have shut down. Once those jets stop supplying energetic particles to the galaxy's large radio lobes, the particles already inside the lobes gradually lose energy, and the radio emission becomes fainter.

Tracking Fading Radio Galaxies

To study these objects in detail, the team combined sensitive observations from the MeerKAT MIGHTEE survey and the uGMRT superMIGHTEE survey with additional measurements from LOFAR, GMRT and the Jansky Very Large Array.

Together, the observations covered radio frequencies from 144 MHz to 1.5 GHz. This broad frequency range allowed researchers to examine how the radio spectra of the galaxies change as particles inside their lobes grow older.

Detailed spectral modeling showed that 12 of the 14 candidates were genuine remnant radio galaxies. The remaining two were instead identified as active sources. The result highlights why extensive observations across multiple radio frequencies are important, since classifications based on a more limited set of radio measurements can produce misleading results.

Surprisingly Young Galactic Remnants

One of the study's most notable findings was how young many of the confirmed remnants appear to be.

Their total spectral ages range from approximately 8-42 million years, with a median age of about 12 million years. That is younger than many remnant radio galaxies examined in previous studies.

The result suggests that astronomers may now be detecting a population of relatively short-lived remnants that earlier surveys largely overlooked.

The objects also represent very different points in the remnant stage of radio galaxy evolution. The proportion of each galaxy's total lifetime spent in the remnant phase ranges from about 4 to 83 percent. Some systems therefore appear to have shut off their jets only recently, while others have been evolving without active jets for much longer.

Distant Galaxies May Fade Faster

Many of the galaxies are also located at relatively high redshifts, meaning they are observed at large cosmic distances.

Under these conditions, relativistic electrons can lose energy more quickly as they interact with the cosmic microwave background. Researchers found a significant negative relationship between redshift and spectral age.

That connection supports the idea that more distant remnant radio galaxies may fade faster, reducing the amount of time during which astronomers can detect them.

Maps showing spectral ages across individual galaxies also revealed that their radio lobes continue changing after the jets have switched off. In extended sources, researchers observed systematic age gradients that are consistent with the movement of plasma through the lobes.

Compact remnants, by comparison, showed less orderly aging patterns. Those differences may be shaped by the galaxies' surrounding environments and the structure of their magnetic fields.

A New View of the Radio Galaxy Life Cycle

Finding these faint and relatively young remnants gives astronomers another piece of the puzzle surrounding the full life cycle of radio galaxies. The results may also improve understanding of the duty cycles of the supermassive black holes at their centers, including when those black holes produce powerful jets and when that activity stops.

The discoveries could also be an early glimpse of a much larger population. Future deep Square Kilometre Array (SKA) radio continuum surveys are expected to uncover many more faint and high-redshift remnant radio galaxies.

The study, "SuperMIGHTEE: Spectral Ages of Remnant Radio Galaxy Candidates in the XMM-LSS Field," is published in the Monthly Notices of the Royal Astronomical Society, Volume 550 (2026).