Synopsis: A Little Empty Inside

A new model has allowed researchers to test a theory for why the centers of dark matter halos are less dense than expected.
Synopsis figure
Manoj Kaplinghat/University of California, Irvine

Every galaxy sits within a blob, or halo, of dark matter, whose distribution can be inferred from its gravitational effects. The most widely used dark matter models can roughly explain the structure of these halos, but they predict that the halos of many small galaxies should be denser at their centers than they actually are. Manoj Kaplinghat at the University of California, Irvine, Sean Tulin at York University in Canada, and Hai-Bo Yu at the University of California, Riverside, have now shown that so-called “self-interacting dark matter” (SIDM) models can reliably predict the density distribution of a wide range of astrophysical halos.

The simplest models portray dark matter as being made of particles that do not interact with each other or with normal matter except through gravity. But in 2000, researchers theorized that if dark matter could interact with itself, the halo density anomaly could be explained. In the high-density cores of galactic halos, dark matter particles would collide and scatter, heating up the inner halo and sending matter outwards. Yet the idea has proven difficult to test against available observations.

Kaplinghat et al. have developed a model that simplifies the calculation of the density distribution of SIDM halos, and they used it to perform the most comprehensive comparison to date between SIDM predictions and observational data. The authors showed that their model could reliably fit data on a variety of observed astrophysical halos—ranging from small dwarf galaxies to galaxy clusters that are 100 times larger. Using their fits, they also derived stringent constraints on parameters characterizing the hypothetical SIDM particles, such as the cross section for the interaction and its dependence on particle velocity.

This research is published in Physical Review Letters.

–Matteo Rini


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