Direct formation of supermassive black holes via multi-scale gas inflows in galaxy mergers

Previously published models of supermassive black hole formation have struggled to explain the fact that — according to observations of distant quasars — supermassive black holes were already in place less than a billion years after the Big Bang. A new series of numerical simulations suggests that the conditions for direct collapse into a supermassive black hole can arise naturally on this time scale from mergers between massive protogalaxies. Multi-scale gas inflows give rise to an unstable, massive nuclear gas disk that expands to form a sub-parsec scale gas cloud in only 100,000 years. The cloud undergoes gravitational collapse, which leads to the formation of a massive black hole. Observations of distant quasars indicate that billion-solar-mass supermassive black holes existed less than a billion years after the Big Bang, but models have struggled to explain this. These authors report simulations showing that mergers between massive protogalaxies produce the conditions for collapse into supermassive black holes. Merger-driven gas inflows give rise to a nuclear gas disk that funnels gas to a sub-parsec-scale cloud. Gravitational collapse of this cloud leads to the formation of a massive black hole. Observations of distant quasars indicate that supermassive black holes of billions of solar masses already existed less than a billion years after the Big Bang1. Models in which the ‘seeds’ of such black holes form by the collapse of primordial metal-free stars2,3 cannot explain the rapid appearance of these supermassive black holes because gas accretion is not sufficiently efficient4,5,6. Alternatively, these black holes may form by direct collapse of gas within isolated protogalaxies7,8, but current models require idealized conditions, such as metal-free gas, to prevent cooling and star formation from consuming the gas reservoir9,10,11. Here we report simulations showing that mergers between massive protogalaxies naturally produce the conditions for direct collapse into a supermassive black hole with no need to suppress cooling and star formation. Merger-driven gas inflows give rise to an unstable, massive nuclear gas disk of a few billion solar masses, which funnels more than 108 solar masses of gas to a sub-parsec-scale gas cloud in only 100,000 years. The cloud undergoes gravitational collapse, which eventually leads to the formation of a massive black hole. The black hole can subsequently grow to a billion solar masses on timescales of about 108 years by accreting gas from the surrounding disk.