Estimate of dark halo ellipticity by lensing flexion

TL;DR

This paper proposes a novel method using galaxy-galaxy flexion to directly measure the ellipticity of dark matter halos, offering a new way to test structure formation models.

Contribution

It introduces a flexion-based estimator for halo ellipticity, tests it with simulations, and discusses bias correction and alternative fitting methods.

Findings

The flexion ratio estimator can measure halo ellipticity.

The estimator shows a slight bias that can be corrected.

Parametric fitting improves accuracy for individual lenses.

Abstract

Aims. The predictions of the ellipticity of the dark matter halos from models of structure formation are notoriously difficult to test with observations. A direct measurement would give important constraints on the formation of galaxies, and its effect on the dark matter distribution in their halos. Here we show that galaxy-galaxy flexion provides a direct and potentially powerful method for determining the ellipticity of (an ensemble of) elliptical lenses. Methods. We decompose the spin-1 flexion into a radial and a tangential component. Using the ratio of tangential-to- radial flexion, which is independent of the radial mass profile, the mass ellipticity can be estimated. Results. An estimator for the ellipticity of the mass distribution is derived and tested with simulations. We show that the estimator is slightly biased. We quantify this bias, and provide a method to reduce it. Furthermore, a parametric fitting of the flexion ratio and orientation provides another estimate for the dark halo ellipticity, which is more accurate for individual lenses Overall, galaxy-galaxy flexion appears as a powerful tool for constraining the ellipticity of mass distributions.