Constraining Self-Interacting Dark Matter with Cluster Mergers

The Musket Ball Cluster (see below) led me to appreciate the importance of galaxy cluster mergers as laboratories for finding self-interacting dark matter. We know remarkably little about dark matter: it could interact with itself at the level of the strong nuclear force and we wouldn't know it yet. Merging clusters are basically dark matter colliders---something we can't build on Earth---which will tell us the self-interaction cross section. Here's the basic story of a cluster merger in the standard case of no self-interaction:

where the dots represent galaxies, blue represents the dark matter, and red represents gas (purple is a mixture of blue and red). Not all mergers occur have the right conditions to make the dissociation between gas and dark matter so clear, but some do. Now here's the story with self-interacting dark matter:

The self-interaction of the dark matter manifests itself as a slight lag of the dark matter behind the galaxies. My group is vigorously pursuing observations of an ensemble of mergers to detect (or place an upper limit on) any galaxy/dark matter separation. See the Merging Cluster Collaboration for more details and some nice pictures.

Discovery of a Merging Galaxy Cluster

The Deep Lens Survey team has discovered a merging galaxy cluster in which the galaxies and dark matter (colored blue below) have separated from the gas (colored red below). My student Will Dawson led the analysis and explains it on a less technical level on on his website.

Gravitational Lensing

Because mass bends the path of light, we can use observations of background sources of light to infer the presence of foreground masses, as well as their total mass and how it is distributed. Because most matter in the universe is dark, gravitational lensing has become an important astrophysical tool. I was lead author on the discovery of gravitational lensing by large-scale structure and on the first discovery of a galaxy cluster via lensing and the first use of tomography in lensing, and I continue to pursue these topics through the Deep Lens Survey and the planned Large Synoptic Survey Telescope described below. In fact, the merging cluster depicted above was discovered via lensing in the DLS.

Deep Lens Survey

I am currently focused on the Deep Lens Survey (DLS), a deep optical imaging survey of 20 square degrees of sky. As Co-PI, I am deeply involved in all aspects of the survey, from planning and coordinating over 100 nights of 4-meter telescope time, to data processing and algorithms, to final science analysis and public data release. A fun part of this is producing beautiful color images of the sky (more are available on the DLS website).

Large Synoptic Survey Telescope

I am also involved in planning and design for the Large Synoptic Survey Telescope (LSST), an 8-m telescope with a 3-degree field of view which will repeatedly survey the sky beginning in the early 2020's. For astronomers or physicists who would like to know more, I highly recommend the LSST Science Book. (Full disclosure: as then-co-chair of the LSST Weak Lensing Science Collaboration, I co-edited the weak lensing chapter.) For others, try the Public part of the LSST website.