Dying radio galaxies suggest a shorter, more dynamic afterlife for black hole jets

Astronomers have discovered a previously overlooked group of faint, quickly fading radio galaxy remnants, which sheds light on the brief, intense phase following supermassive black hole jets' power-down. Led by researchers from the University of Cape Town and the Inter-University Institute for Data Intensive Astronomy, the study examined 14 candidate remnants within the XMM-LSS field.

The team integrated data from MeerKAT MIGHTEE, uGMRT superMIGHTEE, LOFAR, GMRT, and the Jansky Very Large Array, offering extensive radio coverage from 144 MHz to 1.5 GHz. The multifrequency observations led to the confirmation of 12 out of 14 as genuine remnants, while two were reclassified as active sources. The findings revealed that these remnants have ages ranging from 8 to 42 million years, with an average age of about 12 million years, suggesting they represent a population of short-lived remnants that have been underrepresented in earlier surveys.

The galaxies exhibited varying evolutionary stages, with 4% to 83% of their total lifetimes spent in the remnant phase. Additionally, a significant correlation was found between redshift and spectral age, indicating that higher redshift remnants fade more rapidly and are detectable for shorter periods due to increased energy loss from relativistic electrons interacting with the cosmic microwave background.

The spatially resolved spectral-age maps further showed that radio lobes continue to evolve after the jets have ceased, with compact remnants displaying irregular aging patterns possibly influenced by their environments and magnetic-field structures. This discovery advances our understanding of radio galaxy life cycles and the activity cycles of their central supermassive black holes, and hints at the vast population of faint, high-redshift remnants expected to be uncovered by future Square Kilometre Array (SKA) radio continuum surveys.