Non-halo structures and their effects on gravitationally lensed galaxies

TL;DR

This study examines how non-halo dark matter structures like filaments influence gravitational lensing, finding their effects are subtle and often absorbed by standard models, especially in colder dark matter scenarios.

Contribution

It introduces a Bayesian approach to quantify non-halo dark matter effects on galaxy-galaxy lensing using Warm Dark Matter simulations, highlighting their subtle impact.

Findings

Non-halo structures cause minor shifts in lens parameters.

Effects are absorbed by standard lens models, making them hard to detect.

Impact is minimal in colder WDM and CDM scenarios.

Abstract

While the $\Lambda$CDM model succeeds on large scales, its validity on smaller scales remains uncertain. Recent works suggest that non-halo dark matter structures, such as filaments and walls, could significantly influence gravitational lensing and that the importance of these effects depends on the dark matter model: in warm dark matter scenarios, fewer low-mass objects form and thus their mass is redistributed into the cosmic-web. We investigate these effects on galaxy-galaxy lensing using fragmentation-free Warm Dark Matter (WDM) simulations with particle masses of m$_{\chi}$ = 1 keV and m$_{\chi}$ = 3 keV. Although these cosmological scenarios are already observationally excluded, the fraction of mass falling outside of haloes grows with the thermal velocity of the dark matter particles, which allows for the search for first-order effects. We create mock datasets, based on gravitationally-lensed systems from the BELLS-Gallery, incorporating non-halo contributions from these simulations to study their impact in comparison to mocks where the lens has a smooth mass distribution. Using Bayesian modelling, we find that perturbations from WDM non-halo structures produce an effect on the inferred parameters of the main lens and shift the reconstructed source position. However, these variations are subtle and are effectively absorbed by standard elliptical power-law lens models, making them challenging to distinguish from intrinsic lensing features. Most importantly, non-halo perturbation does not appear as a strong external shear term, which is commonly used in gravitational lensing analyses to represent large-scale perturbations. Our results demonstrate that while non-halo structures can affect the lensing analysis, the overall impact remains indistinguishable from variations of the main lens in colder WDM and CDM scenarios, where non-halo contributions are smaller.