Through a Smoother Lens: An expected absence of LCDM substructure detections from hydrodynamic and dark matter only simulations

摘要

A fundamental prediction of the cold dark matter cosmology is the existenceof a large number of dark subhalos around galaxies, most of which should beentirely devoid of stars. Confirming the existence of dark substructures standsamong the most important empirical challenges in modern cosmology: if they arefound and quantified with the mass spectrum expected, then this would close thedoor on a vast array of competing theories. But in order for observationalprograms of this kind to reach fruition, we need robust predictions. Here weexplore substructure predictions for lensing using galaxy lens-like hosts atz=0.2 from the Illustris simulations both in full hydrodynamics and dark matteronly. We quantify substructures more massive than ~ 10^9 M_sun, comparable tocurrent lensing detections derived from HST, Keck, and ALMA. The addition offull hydrodynamics reduces the overall subhalo mass function by about a factorof two. Even for the dark matter only runs, most (~ 85%) lines of sight throughprojected cylinders of size close to an Einstein radius contain nosubstructures larger than 10^9 M_sun. The fraction of empty sight lines risesto ~ 95% in full physics simulations. This suggests we will likely needhundreds of strong lensing systems suitable for substructure studies, as wellas predictions that include the effects of baryon physics on substructure, toproperly constrain cosmological models. Fortunately, the field is poised tofulfill these requirements.