Dark Concentration -- It Matters!

By Andrea Albert

In the hunt for dark matter, any information to help us narrow in on what to look for is key.  Miguel Sánchez-Conde (KIPAC and Stockholm University) and Francisco Prada (IFT/UAM, Madrid) have just published a crucial clue, concerning the concentration of dark matter halos, which are self-gravitating accumulations of dark matter that host systems like galaxies and galaxy clusters.  Their recent paper on “the flattening of the concentration-mass relation towards low halo masses and its implications for the annihilation signal boost” combines theoretical predictions with simulations to learn about Earth-mass to galaxy cluster-sized halos.  “The point of the paper is to put together the theory and the [simulations],” says Sánchez-Conde.

Dark matter is the gravitational glue that we know must exist to hold systems like galaxies and galaxy clusters together.  Astrophysicists, including myself, are searching for “indirect signatures” of dark matter interactions in the Universe.  Instead of waiting for the dark matter to interact with our detector here on Earth, we look for the by-products of dark matter particles colliding with each other out in the Universe.  The brightness of our expected signal depends on both the probability of interacting if they come across each other and on how often they meet.  You can imagine in a concentrated, or crowded, region of dark matter you would expect lots of interactions, and therefore a bright annihilation signal.

Computer simulation of the dark matter in a cluster of galaxies, showing numerous structures with a wide variety of masses. Visualization: Ralf Kaehler;  Simulation: Hao-Yi Wu, Oliver Hahn, Risa Wechsler (KIPAC).

The concentration in dark matter halos of all possible predicted masses is what Sánchez-Conde and Prada were investigating.  They were able to use information coded in the currently-favored “cold dark matter” (CDM) cosmological framework to figure out how concentrated dark matter halos are at different mass scales.  In developing the theoretical prediction, Sánchez-Conde said they were “listening to what the CDM paradigm has to say about the structural halo properties.”  When they compared their predictions to detailed simulations of how dark matter is expected to evolve over the lifetime of the Universe, and at different mass scales, they found they were a good match.  It seems the concentration of dark matter halos eventually starts to level off at lower masses, instead of continuing to increase at the rate seen at higher masses, as was previously thought.  This has huge implications for the expected brightness of dark matter signals from objects such as galaxy clusters, which are composed of millions of subhalos with a variety of masses.  Knowing how bright a signal you expect from each clump allows you to get a better handle on what to expect from the galaxy cluster as a whole.

Plot of the (logarithms of the) concentration (c200) vs. dark matter halo mass (M200) vs. at the present time.  Solid black line + grey band show theoretical prediction for the CDM paradigm.  Data points are from various dark matter simulations. Sánchez-Conde and Prada (2014)

There are still many unanswered questions: we are missing numerical simulations of medium-scale dark matter halos, with masses between those of large planets and small galaxies.  Sánchez-Conde and collaborators are already working on this.  “We are running simulations [with] supercomputers around the world to fill in the gap,” he explains.  Though the hunt for dark matter continues, we have made progress in understanding how concentrated dark matter is at various scales, which tells us how bright a signal we should expect, if the dark matter is really CDM.  

Every new clue is important and brings us one step closer to solving the dark matter mystery.

You can read the paper (Sanchez-Conde & Prada 2014) on the arxiv here.

See this link for many more graphically impressive simulation images and video clips from the KIPAC Astrophysics Visualization Group.