A unique insight into the nature of dark matter has been revealed by studies of how it interacts with supermassive black holes at the centres of galaxies. Dr Xavier Hernandez and Dr William Lee of the National Autonomous University (UNAM) of Mexico have found that some property of dark matter is preventing too much of it from concentrating in galactic centres. If it were allowed to concentrate there it would lead to black holes gobbling up too much of the dark matter in a galaxy, fundamentally altering the structure of the surrounding galaxy.

The first large structures to appear in the Universe following the big bang were massive clouds, or haloes, of dark matter, whose gravity was sufficient to drag inwards the requisite hydrogen gas for building a galaxy inside the dark halo. Consequently, all galaxies come enveloped in an unseen cushion of dark matter, which we can only detect through its gravitational influence.

Theory had always predicted that the dark matter would come in the shape of a ‘cuspy’ halo; in other words, the deeper the dark matter penetrates into the core of a galaxy, the thicker the cloud becomes as the density increases. However, when Hernandez and Lee modelled this, they found that the black hole would begin consuming too much dark matter, leading to a runaway process that relatively quickly (cosmologically speaking) would cause the dark matter halo to become distorted, dramatically changing a galaxy’s appearance.

“[The galaxy] would show a highly concentrated stellar population orbiting not within a dark matter halo but around a black hole with a large mass than anything inferred to date,” say the two researchers.

Evidently galaxies do not look at all like this, so Hernandez and Lee postulate that the density of the dark matter halo must be fairly constant towards the central regions to moderate the amount of dark matter being swept up by the black hole. They find that for the largest black holes in quasars, with masses of five billion times the mass of the Sun, the dark matter density must not account for more than 250 solar masses per parsec cubed (about seven solar masses per cubic light year) to resist runaway absorption by the black hole.

Gravity is the only force that, at present, has been found to work on dark matter particles, and so one would expect that the mutual gravity of dark matter particles would cause the halo to have a higher density at its centre. That in practice it does not, says Hernandez and Lee, means that there may be some kind of internal force acting upon dark matter particles, “making it hard to pack too much in a small volume and resulting in flatter density profiles, even if the interaction with normal matter remains only gravitational,” Lee and Hernandez tell Astronomy Now. Although at first glance this only adds another layer of mystery onto dark matter, it could provide new constraints as to what kind of particle dark matter is.