Supermassive black holes are thought to be lurking at the centre of almost every large galaxy, but very little is known about their origins. Current thinking suggests that these cosmic behemoths, which can have masses millions of times the mass of the Sun, would grow relatively slowly as it would take a considerable amount of time for them to suck in the matter that surrounds them.
However, there must be something else going on as several supermassive black holes have been observed that date back to the early days of the Universe.
Now, a team at physicists at the University of California think they may have an explanation: seed black holes, or black holes in their initial stages, can be formed by the collapse of the dark matter halos that surround galaxies.
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“Physicists are puzzled why supermassive black holes in the early Universe, which are located in the central regions of dark matter halos, grow so massively in a short time,” said study leader Hai-Bo Yu, an associate professor of physics and astronomy at UC Riverside, who led the study.
“It’s like a five-year-old child that weighs, say, 200 pounds (90kg). Such a child would astonish us all because we know the typical weight of a new-born baby and how fast this baby can grow.
“Where it comes to black holes, physicists have general expectations about the mass of a seed black hole and its growth rate. The presence of supermassive black holes suggests these general expectations have been violated, requiring new knowledge. And that’s exciting.”
According to Yu’s theory, the halo is initially formed as gravity pulls particles of dark matter closer and closer together. As the halo forms, a battle begins between gravity, which is pulling the particles into the centre of the halo, and pressure, which is pushing them outwards.
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If the dark matter particles cannot interact with one another then they would heat up, increasing the outward pressure and preventing the halo from collapsing. If they can interact with another, however, the heat could be evenly distributed throughout the particles leading to the halo’s eventual collapse and the formation of a seed black hole.
This seed can then grow more massive by sucking in any surrounding baryonic, or visible, matter such as gas and stars.
“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. Thus, it can grow into a supermassive black hole in a relatively short timescale,” said Yu.
“In many galaxies, stars and gas dominate their central regions. Thus, it’s natural to ask how the presence of this baryonic matter affects the collapse process. We show it will speed up the onset of the collapse. This feature is exactly what we need to explain the origin of supermassive black holes in the early Universe.”