Supermassive black holes are the most energetic sources in the Universe and reside at the centre of most massive galaxies. They have more than a million times the mass of the Sun and grow through the accretion of matter and via coalescences during galaxy mergers.

When they are actively accreting as active galactic nuclei (AGN) the surrounding matter that feeds them can radiate over a trillion times the energy of the Sun. The growth of supermassive black holes is thought to be linked to that of their host galaxies, a co-evolution that can be explained if a fraction of the AGN energy output (AGN feedback) couples with the galactic medium and regulates star formation (and thus the growth of the host galaxy).

Despite the key role supermassive black holes are thought to play in galaxy formation and evolution, their origin is still a major challenge of modern astrophysics. The discovery that supermassive black holes already existed in the early Universe (700 million years after the Big Bang) puzzles the astronomical community. How did such massive black holes have time to form in such a young Universe? To reach that mass in such a short time they should have started as lower-mass seed black holes at even earlier epochs. Observing these seed black holes in the early Universe is difficult; however, evidence for their existence can be found in the nucleus of local dwarf galaxies that have not evolved much with time.