When the jets of a supermassive black hole fall silent, a radio galaxy enters a glowing afterlife – and that phase appears far shorter and more turbulent than assumed. So reports a team led by Sushant Dutta of the University of Cape Town and the IDIA institute in the journal Monthly Notices of the Royal Astronomical Society.
So-called remnant radio galaxies form when a galaxy’s active core switches off and its giant radio lobes are no longer fed with fresh energy. They are a key tool for measuring the on-and-off duty cycles of active galactic nuclei – but they are notoriously hard to detect.
Twelve confirmed remnants
The team studied 14 candidates in the deep XMM–LSS survey field. They combined data from South Africa’s MeerKAT telescope (the MIGHTEE project), India’s upgraded GMRT (superMIGHTEE), plus LOFAR, the GMRT and the Jansky Very Large Array – together spanning 144 MHz to 1.5 GHz. Spectral modelling confirmed twelve objects as genuine remnants; two turned out to be still active.
Higher redshift, shorter glow
The measured spectral ages range from about 8 to 42 million years, with a median near twelve million – remarkably brief. A clear trend stands out: the more distant and thus redshifted a galaxy is, the younger its remnant. The researchers attribute this to the cosmic microwave background, against which relativistic electrons lose energy faster at greater distances.
- 14 candidates, 12 confirmed as remnants
- Spectral ages roughly 8–42 million years (median about 12)
- Redshift z = 0.35 to 2.85 (median 1.25)
What it means for black holes
Distant remnants therefore stay visible only briefly, which skews their census and affects estimates of how long the jets live. Future, more sensitive surveys with MeerKAT and the Square Kilometre Array now under construction should reveal many more of these fading giants.
Sources: Monthly Notices of the Royal Astronomical Society · arXiv · EurekAlert (University of Cape Town)
