Observed versus Simulated Halo c-Mvir Relations

TL;DR

This study compares observed and simulated dark matter halo concentration-mass relations, finding significant discrepancies that suggest possible sample biases or the need for revised models of halo evolution.

Contribution

It introduces a robust bootstrapping method combining stellar and lensing profiles and assesses systematic differences using EAGLE simulations.

Findings

Observed Milky Way-sized halos have higher concentrations than simulations.

Discrepancies are robust against methodological variations.

Potential explanations include sample selection bias or different halo assembly histories.

Abstract

The concentration - virial mass relation is a well-defined trend that reflects the formation of structure in an expanding Universe. Numerical simulations reveal a marked correlation that depends on the collapse time of dark matter halos and their subsequent assembly history. However, observational constraints are mostly limited to the massive end via X-ray emission of the hot diffuse gas in clusters. An alternative approach, based on gravitational lensing over galaxy scales, revealed an intriguingly high concentration at Milky Way-sized halos. This letter focuses on the robustness of these results by adopting a bootstrapping approach that combines stellar and lensing mass profiles. We also apply the identical methodology to simulated halos from EAGLE to assess any systematic. We bypass several shortcomings of ensemble type lens reconstruction and conclude that the mismatch between observed and simulated concentration-to-virial-mass relations are robust, and need to be explained either invoking a lensing-related sample selection bias, or a careful investigation of the evolution of concentration with assembly history. For reference, at a halo mass of $10^{12} M_\odot$, the concentration of observed lenses is $c_{12}\sim 40\pm 5$, whereas simulations give $c_{12}\sim 15\pm1$.