Supermassive black holes are thought to grow slowly, as they pull in matter from around them over billions of years. But if that’s the case, how did they appear so early in the universe’s lifetime? Now, researchers have proposed a new origin story for them – black hole seeds created by the collapse of halos of self-interacting dark matter.
Sporting millions or billions of times the mass of the Sun, supermassive black holes lurk at the centers of most galaxies. Currently, the most widely accepted story for how they came to be is that pockets of primordial gas collapsed into smaller black hole “seeds,” which grew over billions of years as they merged with other black holes, swallowed stars or slurped up dust, gas and anything else that wanders too close.
But recent discoveries have poked holes in that hypothesis. More and more supermassive black holes have been discovered at great distances, indicating they were already well-established within the first few hundred million years after the Big Bang. That shouldn’t have given them enough time to grow so big, according to the existing model.
So astronomers have proposed other stories. One recent study suggested that the early supermassive black holes were actually created after colossal primordial stars, with tens of thousands of times the mass of the Sun, went supernova.
In this latest study, a team led by researchers at the University of California, Riverside, developed a new idea. It’s built around dark matter, the mysterious stuff that’s thought to make up the bulk of the matter in the universe and holds galaxies together with its gravitational pull.
Conventional thinking goes that dark matter would naturally cluster into halos throughout the universe. And, the UCR team says, under certain circumstances these dark matter halos could eventually become dense enough that they collapse under their own gravity, forming a black hole seed.
The key is that in this scenario, dark matter would need to be “self-interacting.” If dark matter particles can interact with each other, the researchers say that the collapsing halo would shrink and create a viscosity that slows its spin. Eventually that would trigger a black hole seed, which in turn can grow fairly quickly by feeding on regular matter in the area, like dust and gas.
“The advantage of our scenario is that the mass of the seed black hole can be high since it is produced by the collapse of a dark matter halo,” says Hai-Bo Yu, lead researcher on the study. “Thus, it can grow into a supermassive black hole in a relatively short timescale.”
Self-interacting dark matter is a fairly convenient explanation for the mystery of fast-growing supermassive black holes, but it does have a few caveats. The major one is that the most widely accepted model says that dark matter doesn’t interact with itself – but it’s not completely outside the realm of possibility. Many models do allow for this kind of interaction, and indeed astronomers have found evidence that may back it up.
Either way, it’s an intriguing story, and one that may become more or less favored as other tidbits come to light.
The research was published in the Astrophysical Journal Letters.