KIPAC Tea Talk: Are Cluster Galaxies Too Efficient at Strong Lensing? Revisiting a Tension Between Observations and Simulations / Exploring a cosmic ray inverse-Compton origin to the SZ-to-X-ray pressure deficit in the cool core cluster ZwCl 3146

Roche 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.

Silich Abstract: Our group has recently proposed the possibility that inverse-Compton scattering of CMB photons by ~GeV cosmic rays (CR-IC) injected by central AGN in cool core galaxy clusters produces a non-negligible continuum-like X-ray signal that is easily misinterpreted as ICM thermal bremsstrahlung continuum. This is particularly relevant to the cooling flow problem—the lack of star formation relative to X-ray-inferred ICM cooling rates. We have explored this possibility in the ZwCl 3146 cluster by comparing pressure profiles derived via X-rays and the thermal SZ effect, since while SZ measurements probe only thermal ICM electrons, additional CR–IC emission would appear to boost the X-ray–inferred pressure. In this talk, I will provide an overview of our results, which establish that non-negligible CR-IC emission is plausible in ZwCl 3146, and discuss future detailed work needed to robustly assess whether CR-IC could be relevant to the cooling flow problem.