Slow stellar collisions built early supermassive black holes

Slow stellar collisions in UCLA's simulations merged stars in ultra-dense early clusters into one star as massive as its whole cluster. When that star reached the end of its life, it collapsed into a massive black hole, a seed for the supermassive black holes that formed shortly after the Big Bang.

Source: Dong-A Science
Image credit: UCLA simulation, via Dong-A Science

The problem this tries to solve is a timing problem. Webb and other telescopes keep finding supermassive black holes — millions and billions of times the Sun's mass — less than a billion years after the Big Bang. Growing a black hole that big takes time. A black hole born from one collapsed star has to eat steadily, near the fastest rate physics allows, for nearly the whole age of the universe at that point, and it still barely makes it. Something must have given the process a head start.

The UCLA simulations offer one head start: start bigger. In an ultra-dense young star cluster, stars are packed so tightly that collisions are common. When two stars meet slowly, they merge instead of shattering, and the merged star is bigger. Repeat that over and over, and runaway growth builds a single monster star as massive as its whole cluster. A star that massive lives fast and dies by collapsing straight into a black hole — a seed far heavier than any single star could leave behind. From there, ordinary growth has a much shorter road to supermassive.

Slow is the key word. Fast collisions tear stars apart. Slow ones stick. The early universe made dense clusters more easily than the present day does, which is why this path belongs to that era specifically.

This is a simulation result, not an observation. Nobody has watched a runaway merger build a monster star. The test will be finding the middleweights: black holes heavier than any single star should leave behind, but lighter than the supermassive ones — the missing middle of the black hole family. Astronomers are hunting them now, in dense clusters and in the gravitational-wave signals of mergers. If the middleweights are there, the collision path gains real weight. If they are not, the timing problem stays open.