Turbulent cold flows gave birth to the first quasars.

How quasars powered by supermassive black holes formed less than a billion years after the Big Bang is still one of the outstanding problems in astrophysics, 20 years after their discovery. Cosmological simulations suggest that rare cold flows converging on primordial haloes in low-shear environments could have created these quasars if they were 10-10 solar masses at birth, but could not resolve their formation. Semi-analytical studies of the progenitor halo of a primordial quasar found that it favours the formation of such seeds, but could not verify if one actually appeared. Here we show that a halo at the rare convergence of strong, cold accretion flows creates massive black holes seeds without the need for ultraviolet backgrounds, supersonic streaming motions or even atomic cooling. Cold flows drive violent, supersonic turbulence in the halo, which prevents star formation until it reaches a mass that triggers sudden, catastrophic baryon collapse that forms 31,000 and 40,000 solar-mass stars. This simple, robust process ensures that haloes capable of forming quasars by a redshift of z > 6 produce massive seeds. The first quasars were thus a natural consequence of structure formation in cold dark matter cosmologies, and not exotic, finely tuned environments as previously thought.