[Paper Review] Halo concentration strengthens dark matter constraints in galaxy-galaxy strong lensing analyses

J.W Nightingale, Qiuhan He+4 more|arXiv (Cornell University)|Dec 7, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy67 references40 citations

TL;DR

This paper demonstrates that halo concentration significantly enhances the detectability of low-mass dark matter haloes in galaxy-galaxy strong lensing, making constraints on warm dark matter (WDM) particle mass up to an order of magnitude more stringent than previously thought. By simulating lensed images and accounting for the mass-concentration relation, the study shows that high-concentration haloes—common in WDM models—produce stronger, more detectable lensing perturbations, even when their abundances are suppressed.

ABSTRACT

A defining prediction of the cold dark matter cosmological model is the existence of a very large population of low-mass haloes. This population is absent in models in which the dark matter particle is warm (WDM). These alternatives can, in principle, be distinguished observationally because haloes along the line of sight can perturb galaxy–galaxy strong gravitational lenses. Furthermore, the WDM particle mass could be deduced because the cut-off in their halo mass function depends on the mass of the particle. We systematically explore the detectability of low-mass haloes in WDM models by simulating and fitting mock lensed images. Contrary to previous studies, we find that haloes are harder to detect when they are either behind or in front of the lens. Furthermore, we find that the perturbing effect of haloes increases with their concentration: Detectable haloes are systematically high-concentration haloes, and accounting for the scatter in the mass–concentration relation boosts the expected number of detections by as much as an order of magnitude. Haloes have lower concentration for lower particle masses and this further suppresses the number of detectable haloes beyond the reduction arising from the lower halo abundances alone. Taking these effects into account can make lensing constraints on the value of the mass function cut-off at least an order of magnitude more stringent than previously appreciated.

Motivation & Objective

• To assess the detectability of low-mass dark matter haloes in warm dark matter (WDM) models using strong gravitational lensing.
• To investigate how halo concentration affects the observability of perturbations in lensed images.
• To quantify the impact of the mass-concentration scatter on the number of detectable haloes and resulting WDM constraints.
• To improve predictions of halo detection rates by incorporating realistic halo structural properties into lensing simulations.
• To re-evaluate the sensitivity of current Hubble Space Telescope (HST) lensing data to WDM particle mass by accounting for concentration effects.

Proposed method

• Simulated mock lensed images of galaxy-galaxy strong lensing systems using realistic lens models and cosmological halo populations.
• Applied a Bayesian framework to compute the sensitivity function, defining the probability p of detecting a halo given its properties and lensing system configuration.
• Incorporated the mass-concentration relation and its scatter into the halo population model, with concentrations decreasing for lower WDM particle masses.
• Used the log-likelihood increase (∆L) as a detection metric, with a threshold of ∆L > 30 (quad) or ∆L > 20 (arcs) to define detectability.
• Compared detection expectations between models that include concentration effects and those that assume a fixed or averaged concentration.
• Calculated the likelihood ratio R between WDM models with different cut-off masses and CDM, using Poisson-distributed detection counts.

Experimental results

Research questions

• RQ1 How does halo concentration influence the detectability of low-mass dark matter haloes in strong lensing systems?
• RQ2 To what extent does the scatter in the mass-concentration relation amplify the number of detectable haloes in WDM models?
• RQ3 How do concentration effects alter the expected number of detectable haloes compared to models assuming fixed or average concentrations?
• RQ4 What is the impact of halo concentration on the sensitivity of strong lensing to WDM particle mass, especially below the detection threshold of current data?
• RQ5 Can current HST data place more stringent constraints on WDM than previously thought when concentration effects are properly accounted for?

Key findings

• Halo concentration strongly enhances detectability: high-concentration haloes produce more significant lensing perturbations and are therefore more likely to be detected.
• Accounting for the scatter in the mass-concentration relation increases the expected number of detectable haloes by up to an order of magnitude compared to models assuming fixed concentrations.
• WDM haloes have lower concentrations at lower particle masses, which further suppresses detectability beyond the suppression from reduced halo abundances alone.
• When concentration effects are included, WDM constraints on the halo mass function cut-off (Mcut) become at least an order of magnitude more stringent than previously estimated.
• Even with current HST data, which struggle to detect haloes below ∼10^9.5 M⊙, concentration effects allow constraints on WDM cut-off masses as low as Mcut ≳ 10^8 M⊙, effectively probing WDM models one order of magnitude below the nominal sensitivity limit.
• The likelihood ratio analysis shows that models ignoring concentration effects severely underestimate the constraining power of lensing data, especially for low-Mcut WDM scenarios.