Astronomers Find a Population of Radio Galaxies That Fade Unusually Fast

Astronomers have identified a little-studied population of faint remnant radio galaxies that appear to fade quickly once the supermassive black holes at their centres stop producing radio jets. Researchers from the University of Cape Town (UCT) and the Inter-University Institute for Data Intensive Astronomy (IDIA) led the study, examining 14 possible remnant radio galaxies in the XMM-Newton Large-Scale Structure field. In these systems, once jets from the central active galactic nucleus shut down, particles already inside the galaxy's radio-emitting lobes gradually lose energy and the radio signal fades.

The team combined observations from the MeerKAT MIGHTEE and uGMRT superMIGHTEE surveys with additional measurements from LOFAR, GMRT and the Jansky Very Large Array, covering radio frequencies from 144 MHz to 1.5 GHz. Detailed spectral modelling confirmed 12 of the 14 candidates as genuine remnants, while the other two turned out to be still-active sources — showing why observations across many frequencies matter, since more limited data can be misleading.

The confirmed remnants were surprisingly young: their spectral ages ranged from about 8 to 42 million years, with a median of roughly 12 million years, younger than many remnants found in earlier surveys. Many of the galaxies also sit at large cosmic distances, and the researchers found a significant relationship between that distance and spectral age — more distant remnants appear to fade faster, since their electrons lose energy more quickly interacting with the cosmic microwave background, shrinking the window in which astronomers can detect them.

Maps of spectral ages within individual galaxies showed their radio lobes keep changing after the jets switch off, with more extended sources showing orderly age gradients and compact remnants ageing less predictably. The findings, published in Monthly Notices of the Royal Astronomical Society, may help astronomers understand the duty cycles of supermassive black holes, and researchers expect future Square Kilometre Array surveys to uncover many more faint, distant remnants like these.