Astronomers have long asked: what causes galaxies to stop forming new stars? What causes them to transition from blue, star-forming systems to red and dead galaxies? One mechanism often favoured by cosmological simulations that attempt to reproduce the observed Universe is the energy released when matter falls onto the supermassive black holes at the centres of galaxies. When these black holes enter active phases – known as active galactic nuclei (AGN) – the energy they release can heat and expel gas away in the form of winds, preventing the gas from cooling and forming new stars
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