An international team of astronomers has discovered a previously unseen population of galaxies in which the radio lobes show a clear decline in brightness. These lobes are normally the visible signature of powerful jets launched from the vicinity of a supermassive black hole, transporting relativistic particles far into the surrounding intergalactic medium.
Radio lobes are formed when the particles in those jets interact with magnetic fields, producing synchrotron radiation detectable at radio wavelengths. As long as the black hole continues to accrete matter and feed energy into the jets, the lobes remain luminous and extended. When the accretion flow diminishes, the energy supply drops and the particles begin to lose energy through radiation and adiabatic expansion.
The observed fading of the lobes indicates that the engines driving these jets – the black hole’s accretion activity – have switched off or are operating at a markedly reduced level. This provides a rare, direct view into the phase when an active galactic nucleus (AGN) shuts down and its associated outflows gradually dissipate.
Understanding such shutdown episodes is crucial for models of galaxy evolution, because the feedback between black holes and their host galaxies regulates star formation and influences the galaxy’s growth. Future observations will aim to measure how long these faint phases persist and whether they can later reignite, offering a more complete picture of the duty cycle of black‑hole powered activity.