The researchers used cosmological modeling to reconstruct 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 one episode of star formation followed by a steady rain of star formation, as Garcia expected, there were two major cycles of star birth. Whole swarms of stars flashed like garlands on a Christmas tree.
“The early universe was an incredibly crowded place,” Garcia said. “The gas clouds were denser, stars formed faster, and in those conditions, gravity naturally gathers stars into these tightly coupled systems.”
These clusters were initially scattered throughout the galaxy, but then fell towards the center, like water flowing down a drain. Once there, they combined to create one megacluster called a nuclear star cluster (so named because it lies at the core of the galaxy). The young galactic heart shone with the light of a million suns and may have set the stage for the formation of a supermassive black hole.
Modeling the formation of superdense star clusters.
Making the simulation more accurate than previous ones required a seemingly simple tweak. “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 going at a constant rate as in previous models.”
Using the University of Maryland supercomputer complex. UrgentGarcia did in six months what would have taken 12 years on a MacBook.
Some clouds turned up to 80 percent of their gas into stars—a monstrous rate compared with the 2 percent typically observed in nearby galaxies today. The clouds came to life as clusters of newborn stars held together by mutual gravity, opening a new path for the formation of supermassive black holes in the very early stages of the Universe.
Chicken or egg?
Most galaxies, including our own, are associated with a nuclear star cluster located around a supermassive black hole. But the connection between them was a little unclear: did a monstrous black hole form and then pull stars close, or did the cluster itself create a black hole?
