Astronomers have identified a population of faint remnant radio galaxies that appear to fade relatively quickly after their supermassive black holes stop powering their enormous radio jets. The finding comes from a detailed look at 14 candidate remnants, 12 of which were confirmed as genuine remnants after observations across a wide range of radio frequencies.
Radio galaxies can enter a remnant stage after the jets produced by their central active galactic nuclei switch off. Without a continuing supply of energetic particles, the radio lobes left behind by the jets gradually lose energy and fade.
This fading makes remnants difficult to identify, particularly when observations cover only a limited range of radio frequencies. To examine the process in greater detail, researchers from the University of Cape Town and the Inter-University Institute for Data Intensive Astronomy studied 14 candidate remnant radio galaxies in the XMM–Newton Large-Scale Structure field.
The team combined observations from the MeerKAT MIGHTEE survey and the uGMRT superMIGHTEE survey with data from LOFAR, GMRT and the Jansky Very Large Array. Together, these observations covered radio frequencies from 144 MHz to 1.5 GHz.
That broad frequency coverage allowed the researchers to examine how the radio spectra changed as particles inside the lobes aged.
Detailed spectral modeling confirmed 12 of the 14 candidates as remnant radio galaxies. The other two were reclassified as active sources. The result highlights how classifications based on limited radio observations can be misleading and how observations at multiple frequencies can help distinguish genuine remnants from active galaxies.
Most of the confirmed remnants are relatively young
The confirmed remnants had total spectral ages of about 8 million to 42 million years. Their median age was about 12 million years.
Those ages are shorter than the ages measured for many previously studied remnant radio galaxies. The researchers therefore identified what appears to be a population of relatively short-lived remnants that has largely escaped earlier surveys.
The 12 remnants also occupy a broad range of evolutionary stages. The amount of time each galaxy spends in the remnant phase accounts for roughly 4% to 83% of its total lifetime.
That range includes systems whose jets appear to have switched off relatively recently as well as remnants that have undergone a longer period of fading.
Higher-redshift remnants may fade faster
Many of the sources in the sample are at relatively high redshifts. The study found a significant negative relationship between redshift and spectral age.
At greater cosmic distances, relativistic electrons can lose energy more rapidly through interactions with the cosmic microwave background. The researchers say the relationship supports the possibility that remnant radio galaxies at higher redshifts fade more rapidly and remain detectable for shorter periods.
The researchers also created spatially resolved spectral-age maps to examine how different parts of the radio lobes were aging.
In extended sources, the maps showed systematic age gradients consistent with plasma transport. Compact remnants instead showed more irregular aging patterns, which may be influenced by their environments and magnetic-field structure.
The observations therefore capture remnants at different points after their jets have stopped supplying particles to the radio lobes, from relatively young systems to more evolved ones.
The study was published in Monthly Notices of the Royal Astronomical Society.
