Tightening weak lensing constraints on the ellipticity of galaxy-scale dark matter haloes

Abstract

Cosmological simulations predict that galaxies are embedded into triaxial dark matter haloes, which appear approximately elliptical in projection. Weak gravitational lensing allows us to constrain these halo shapes and thereby test the nature of dark matter. Weak lensing has already provided robust detections of the signature of halo flattening at the mass scales of groups and clusters, whereas results for galaxies have been somewhat inconclusive. Here we combine data from five weak lensing surveys (NGVSLenS, KiDS/KV450, CFHTLenS, CS82, and RCSLenS, listed in order of most to least constraining) in order to tighten observational constraints on galaxy-scale halo ellipticity for photometrically selected lens samples. We constrain fh, the average ratio between the aligned component of the halo ellipticity and the ellipticity of the light distribution, finding fh = 0.303−0.079+0.080 for red lens galaxies and fh = 0.217−0.159+0.160 for blue lens galaxies when assuming elliptical Navarro-Frenk-White density profiles and a linear scaling between halo ellipticity and galaxy ellipticity. Our constraints for red galaxies constitute the currently most significant (3.8σ) systematics-corrected detection of the signature of halo flattening at the mass scale of galaxies. Our results are in good agreement with expectations from the Millennium Simulation that apply the same analysis scheme and incorporate models for galaxy–halo misalignment. Assuming these misalignment models and the analysis assumptions stated above are correct, our measurements imply an average dark matter halo ellipticity for the studied red galaxy samples of ⟨|ϵh|⟩ = 0.174 ± 0.046, where |ϵh| = (1 − q)/(1 + q) relates to the ratio q = b/a of the minor and major axes of the projected mass distribution. Similar measurements based on larger upcoming weak lensing data sets can help to calibrate models for intrinsic galaxy alignments, which constitute an important source of systematic uncertainty in cosmological weak lensing studies.

3D hydrodynamic simulations for the formation of the Local Group satellite planes

“…Brimioulle et al 2013), which so far seems to agree with expectations (Milgrom 2013;Brouwer et al 2017Brouwer et al , 2021. At larger distances from the central galaxy, the EFE from surrounding structures would cause the gravity law to become inverse square and to depart from spherical symmetry (Banik & Zhao 2018a), which may explain some recent observations (Schrabback et al 2021). More detailed calculations would require knowledge of how large-scale structure forms in a Milgromian framework and the resulting EFE on galaxies.…”

The weak lensing radial acceleration relation: Constraining modified gravity and cold dark matter theories with KiDS-1000

“…We note that both the SIS and PPL method assume spherical symmetry, while in simulations DM haloes are found to deviate from sphericity, which could lead to deviations in the lensing rotation curves (Cuddeford 1993). However, the mean ellipticity of haloes is observed to be small ( | | = 0.174 ± 0.046, Schrabback et al 2021). The stacking of thousands of lenses with approximately random orientations further reduces the impact on the lensing signal, which means the halo ellipticity will not significantly change our results.…”

From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity

“…It has been claimed that the weak lensing signal is anisotropic relative to the lens, which in a Newtonian analysis implies a DM halo flattened similarly to the baryonic disc [342]. The anisotropic signal has not been conclusively detected, but there is strong evidence for it over the radial range 45-200 kpc.…”