# SARCS strong-lensing galaxy groups: II - mass-concentration relation and strong-lensing bias

**Authors:** G. Foëx, V. Motta, E. Jullo, M. Limousin, T. Verdugo  
**arXiv:** [1409.5905](https://arxiv.org/abs/1409.5905)  
**Date:** 2014-09-20  
**Summary page:** https://www.summarizepaper.com/en/paper/1409.5905v1/

## Key points

- Study focuses on analysis of 80 strong-lensing galaxy groups using stacked weak-lensing techniques
- Objective is to investigate mass-concentration relation and strong-lensing bias in these systems
- Observed lensing signal compared with singular isothermal sphere (SIS) and Navarro-Frenk-White (NFW) models, neither can be firmly rejected based on lensing signal alone
- Average concentration parameter (c200) derived, value significantly higher than predicted by numerical simulations for corresponding average mass (M200)
- Mass-concentration relation (c(M)) established, spanning nearly two decades in mass, slope disagrees with predictions from simulations using unbiased populations of dark matter haloes
- Authors propose that triaxial haloes preferentially aligned with line of sight could account for observed large concentrations in strong lenses compared to non-lensing haloes
- More massive systems may not require significant elongation along line of sight due to their already large lensing cross sections

## Summary

The study conducted by Foëx et al. focuses on the analysis of a sample of 80 strong-lensing galaxy groups using stacked weak-lensing techniques. The main objective is to investigate the mass-concentration relation and strong-lensing bias in these systems. The authors first compare the observed lensing signal with two different mass distribution models: a simple singular isothermal sphere (SIS) and the expected Navarro-Frenk-White (NFW) profile. The results show that neither model can be firmly rejected based on the lensing signal alone. Next, the study derives the average concentration parameter, denoted as c200, which characterizes the density profile of dark matter haloes. The obtained value, c200 = 8.6_{-1.3}^{+2.1}, is significantly higher than what is predicted by numerical simulations for the corresponding average mass, M200 = 0.73_{-0.10}^{+0.11} x 10^14 solar masses. Furthermore, combining their findings with previous results at larger mass scales, Foëx et al. establish a mass-concentration relation, denoted as c(M), spanning nearly two decades in mass. Surprisingly, this relation exhibits a slope that disagrees with predictions from simulations using unbiased populations of dark matter haloes. To explain these discrepancies, the authors propose that triaxial haloes preferentially aligned with the line of sight could account for the observed large concentrations in strong lenses compared to non-lensing haloes. They suggest that more massive systems do not require significant elongation along the line of sight due to their already large lensing cross sections. Finally, Foëx et al.