Astronomers have identified a little-studied population of faint remnant radio galaxies that appear to fade quickly after their central supermassive black holes stop producing powerful radio jets. The findings provide a closer look at what happens during the final stages of a radio galaxy's evolution. 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. 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. 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.
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