Runaway black hole mergers may have built supermassive black holes
Researchers used cosmological simulations to recreate the first 700 million years of cosmic history, focusing on the formation of a single dwarf galaxy. In their virtual galaxy, waves of stars were born in short explosive bursts as clouds of cold gas collapsed inside a halo of dark matter. Instead of a single episode of star explosion followed by a constant drizzle of star formation as Garcia expected, there were two major cycles of star birth. Whole clouds of stars lit up like Christmas tree lights.
“The early Universe was an incredibly populated place,” Garcia said. “The gas clouds were denser, the stars were forming more quickly, and in these environments it is natural for gravity to bring the stars together in these tightly bound systems.”
These clusters began to be scattered throughout the galaxy, but fell toward the center like water swirling down a sewer. Once there, they merged to create a megacluster, called a nuclear star cluster (so named because it is at the galaxy’s core). The young galactic core glowed with the light of a million suns and could have set the stage for the formation of a supermassive black hole.
A simulation of the formation of super-dense star clusters.
A seemingly simple modification was needed to make the simulation more accurate than previous ones. “Most simulations simplify things to make the calculations more practical, but then you sacrifice realism,” Garcia said. “We used an improved model that allowed star formation to vary depending on local conditions rather than just a constant rate as with previous models.”
Using the University of Maryland’s Zaratan Supercomputing Facility, Garcia achieved in six months what would have taken 12 years on a MacBook.
Some clouds have converted up to 80% of their gas into stars, a ferocious rate compared to the 2% typically seen in nearby galaxies today. The clouds came to life, becoming clusters of newborn stars bound by their mutual gravity and paving the way for the formation of supermassive black holes extremely early in the Universe.
Chicken or egg?
Most galaxies, including our own, are anchored by a nuclear star cluster nestled around a supermassive black hole. But the connection between the two is a bit murky: Did the monstrous black hole form and then bring the stars together, or did the cluster itself give rise to the black hole?
