Thursday

The dark matter particle forms the basis of the standard cosmological model, with the currently favoured candidate cold dark matter (CDM) withstanding extensive observational tests on the Universe's largest scales for over thirty years. However, CDM remains largely untested on smaller scales, for example its prediction of ubiquitous clumps of dark matter with masses 10^9Msun, too low to attract enough baryons to form stars. These clumps do not form in alternative models of warm or self-interacting dark matter. I will discuss how they can be found via galaxy-scale strong gravitational lensing. If dark matter substructures are within the lens galaxy that is deflecting the light of a background source galaxy, they perturb the Einstein ring or lensing arc. Using HST imaging of 43 strong lenses I will infer the presence (or lack thereof) of dark matter clumps in each dataset and discuss how this can constrain the dark matter mass function between masses of 10^8 - 10^10 MSun. This has the potential to rule out many dark matter particle candidates, including CDM.