KIPAC Tea Talk: Are Cluster Galaxies Too Efficient at Strong Lensing? Revisiting a Tension Between Observations and Simulations

Abstract: Cosmological simulations are essential tools for testing models of structure formation and probing the nature of dark matter. When discrepancies arise between simulations and observations, it can indicate the presence of new physics, insufficient complexity of the modeling, or poorly understood systematics. A recently identified tension between Lambda CDM simulations and observations of galaxy clusters is the "Galaxy Galaxy Strong Lensing" tension, in which simulated cluster member galaxies appear to be an order of magnitude less efficient at lensing than their observed counterparts. This suggests that real cluster member galaxies are much more compact than predicted, and hints at exotic dark matter scenarios such as core-collapse self-interacting dark matter. In this talk, I will outline the state of the tension and describe our recent progress in addressing systematic differences in the interpretation of observed and simulated data. This includes the methods used to match properties of the host cluster, measure the mass distribution, and quantify line of sight effects. When accounting for these effects, we still find a clear discrepancy between observed cluster lenses and simulated counterparts, but at significantly lower confidence than previously estimated. I treat this as an example of the extremely useful but subtly challenging practice of comparing observations to simulations, and discuss the role simulations can play in strong lensing inference in the coming era of large scale surveys.