How did supermassive black holes come about? Collapsing halos of darkish matter can clarify them
We don’t fully understand how the first supermassive black holes formed so quickly in the young universe. So a team of physicists comes up with a radical idea. Instead of forming black holes through the usual death-of-a-massive-launch route, giant halos of dark matter collapsed directly, seeding the first large black holes.
Supermassive black holes (SMBHs) occur early in the history of the universe, just a few hundred million years after the Big Bang. This rapid appearance presents a challenge to conventional models of the birth and growth of SMBHs because it doesn’t seem like they can have enough time to grow that massive that quickly.
“Physicists are puzzled as to why SMBHs in the early universe, located in the central regions of dark matter halos, grow so massively in a short period of time,” said Hai-Bo Yu, associate professor of physics and astronomy at UC Riverside who led one Study of SMBH formation that appeared in the Astrophysical Journal Letters. “It’s like a five year old who weighs about 200 pounds. Such a child would astonish us all because we know the typical weight of a newborn and how fast that baby can grow. When it comes to black holes, physicists have general expectations about the mass of a black hole and its rate of growth. The presence of SMBHs suggests that these general expectations have been violated and that new knowledge is needed. And that’s exciting. “
So instead of trying to form black holes from the death of massive stars and then get them to accumulate enough material to go to SMBH status, it may have formed something else – halos of dark matter.
“Our work provides an alternative explanation: a self-interacting halo of dark matter experiences graviothermal instability and its central region collapses into a black seed hole,” said Yu.
There is already a lot of dark matter in the early universe. It makes up over 80% of all matter in the cosmos, and the first galaxies grew in much larger clumps of dark matter, or halos. However, in order for dark matter to collapse enough to form a black hole, it must interact with itself. In this way, it can lose all of the kinetic energy gained when it collapses, allowing it to gain sufficient density to trigger the formation of a black hole.
Since this black hole would be born of much more material than a star, it would already be well on its way to becoming supermassive.
“The advantage of our scenario is that the mass of the seed black hole can be high because it is created by the collapse of a halo of dark matter,” said Yu. “It can grow into a supermassive black hole in a relatively short time.”
In this model, dark matter halos don’t do all of the work. Baryons – normal matter – also help.
“First, we show that the presence of baryons like gas and stars can greatly accelerate the onset of the graviothermal collapse of a halo and a black seed hole could be created soon enough,” said Wei-Xiang Feng, Yu’s graduate student and a co-author on the Paper. “Second, we show that the self-interactions can induce a viscosity that dissipates the residual angular momentum of the central halo. Third, we are developing a method to investigate the condition for triggering the general relativistic instability of the collapsed halo, which ensures that a black hole could form if the condition is met. ”
“In many galaxies, stars and gas dominate their central regions,” said Yu. “So it is natural to ask how the presence of this baryonic matter affects the collapse process. We show that it will accelerate the onset of the collapse. This feature is exactly what we need to explain the origin of supermassive black holes in the early universe. The self-interactions also lead to a viscosity that can reduce the angular momentum of the central halo and further support the collapse process. “