Dark Matter Theory Tested With the Help of Satellite Galaxies in the Milky Way

Dark matter is a non-luminous fabric theorized to account for more than 85 percent of our Universe, but which was not yet demonstrated because it cannot be observed directly. Now, a new study suggests that satellite galaxies in our Milky Way can be utilized to test the basic characteristics of dark matter.

Employing intricate simulations, a team of physicists from the University of California, Riverside, show how a hypothesis that is known as ‘self-interacting dark matter,’ or SIDM, is able to explain various dark matter divisions in Draco and Fornax, two of the Milky Way’s discovered satellite galaxies.

Explaining a Peculiar Phenomenon

The predominant dark matter theory, known as Cold Dark Matter, or CDM, expounds a lot of the Universe, including the way structures take form in it. However, a long-existing difficulty for CDM has been to explain the various dark matter spread in galaxies.

The team of scientists, lead by UC Riverside’s Hai-Bo Yu and Laura V. Sales, analyzed the development of SIDM ‘subhalos’ in the Milky Way ‘tidal field,’ which is the slope in the gravitational field of the Milky Way that a satellite galaxy in which a satellite galaxy falls in the shape of a tidal force. Subhalos are dark matter clusters that house the satellite galaxies.

“We found SIDM can produce diverse dark matter distributions in the halos of Draco and Fornax, in agreement with observations,” said Yu, an associate professor of physics and astronomy and a theoretical physicist with expertise in particle properties of dark matter. “In SIDM, the interaction between the subhalos and the Milky Way’s tides leads to more diverse dark matter distributions in the inner regions of subhalos, compared to their CDM counterparts.”

In their research, the scientists mainly used numerical simulations, known as ‘N-body simulations,’ and got significant results through analytical modeling before testing their simulations.

The Simulations Detected the Right Conditions

Sales detailed how SIDM predicts a phenomenon named ‘core collapse.’ In particular cases, the inner part of the halo crashes under the power of gravity and generates a high density. This is the opposite of the typical expectation that dark matter self-interactions conduct to a low-density halo. The researchers’ simulation detects conditions for the inner part to collapse to take place in subhalos.

“To explain Draco’s high dark matter density, its initial halo concentration needs to be high,” Sales explained. “More dark matter mass needs to be distributed in the inner halo. While this is true for both CDM and SIDM, for SIDM, the core-collapse phenomenon can only occur if the concentration is high so that the collapse timescale is less than the age of the Universe. On the other hand, Fornax has a low-concentrated subhalo, and hence its density remains low.”

The team emphasized that their current work mainly focuses on SIDM and doesn’t make any crucial analysis on the way CDM can explain both Draco and Fornax. After the researchers used numerical simulations to consider the dynamical engagement between dark matter self-interactions and tidal interactions, they discovered something outstanding. The central dark matter of a SIDM subhalo could be enhancing, conflicting with typical expectations. Most importantly, the simulations find conditions for this event to happen in SIDM, and they demonstrate that it can explain observations of Draco.

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