Black holes can form without dark matter

Dark Matter Black Holes?

Can dark matter also collapse into a black hole? According to a new model, this is physically possible and could just happen in galaxy nuclei. If so, then this mechanism could also solve the riddle of the first supermassive black holes in the cosmos. Because they came about before there was enough normal raw material for them.

It is one of the great puzzles in astronomy: According to popular theory, supermassive black holes grow when smaller black holes merge and then gradually devour more and more matter. That takes a lot of time and sufficient material replenishment. But the oldest known quasar and other early giants demonstrate the seemingly impossible: It existed 690 million years after the Big Bang and still comprised 800 million solar masses. How such gravity giants came into being so quickly after the Big Bang is completely puzzling.

Can dark matter also degenerate?

Researchers led by Carlos Argüelles from the La Plata National University in Argentina are now presenting a completely new theory. They used astrophysical models to study how dark matter behaves in the halo of galaxies. The halo forms a spherical area around the main visible plane of the galaxies and contains mostly plasma and dark matter.

Specifically, the astrophysicists wanted to know whether the halo in the center of galaxies can be compressed so much that dark matter degenerates and, in extreme cases, collapses into a black hole. Because the nature of dark matter and thus also of its particles is unknown, Argüelles and his team based their model on fermions of the “warm dark matter” theory. According to this, dark matter consists of weakly interacting particles with a rather low mass, similar to the hypothetical sterile neutrinos.

Possible collapse to black hole

The model calculations showed: Even the dark matter of a halo can theoretically collapse under its own gravity. “In the case of dark matter fermions, a degenerate core then develops, surrounded by a thinned halo,” the researchers report. This structure can remain stable over long periods of time and - viewed from a distance - have characteristics similar to those of a black hole.

But it goes one step further: If the halo is massive enough, its core of dark matter can even collapse completely. "We find a critical point in the core-halo equations at which the fermion core collapses into a supermassive black hole," write Argüelles and his team. As a result, a black hole with several hundred million solar masses is created in a very short time.

Neither star precursors nor much time needed

“This scenario could thus offer a natural explanation for how supermassive black holes were formed in the early universe,” says Argüelles. Because this collapse of dark matter was already possible in the young universe without any problems and neither required a previous star formation nor exotically high accretion rates from smaller predecessor holes, as some hypotheses suggest.

Instead, the first supermassive black holes could have formed in the center of the first galaxies solely through gravity - from their dark matter halo. "We have shown here for the first time that such core-halo distributions are possible within the usual cosmological framework and that they then remain stable over the entire lifetime of the cosmos," the researchers explain.

In her view, her model could be instrumental in understanding the formation of supermassive black holes in the early universe. At the same time, the results could also offer new insights into the formation and development of galaxies and their dense centers. Argüelles and his colleagues hope that future, more precise glimpses into galaxy nuclei will provide further evidence for their theory. (Monthly Notices of the Royal Astronomical Society, 2021; doi: 10.1093 / mnras / staa3986)

Source: Royal Astronomical Society

March 1, 2021

- Nadja Podbregar