Probing Galaxy Dark Matter Haloes in COSMOS with Weak Lensing Flexion

TL;DR

This paper reports the first detection of galaxy-galaxy flexion in space-based data, demonstrating that including flexion measurements alongside shear improves constraints on galaxy dark matter halo density profiles.

Contribution

It introduces a new Shapelets pipeline for flexion detection and shows flexion's added value in constraining dark matter halo profiles in the COSMOS survey.

Findings

First detection of galaxy-galaxy flexion in space-based data

Flexion improves constraints on galaxy density profiles

Average density profile consistent with an isothermal sphere

Abstract

Current theories of structure formation predict specific density profiles of galaxy dark matter haloes, and with weak gravitational lensing we can probe these profiles on several scales. On small scales, higher-order shape distortions known as flexion add significant detail to the weak lensing measurements. We present here the first detection of a galaxy-galaxy flexion signal in space-based data, obtained using a new Shapelets pipeline introduced here. We combine this higher-order lensing signal with shear to constrain the average density profile of the galaxy lenses in the Hubble Space Telescope COSMOS survey. We also show that light from nearby bright objects can significantly affect flexion measurements. After correcting for the influence of lens light, we show that the inclusion of flexion provides tighter constraints on density profiles than does shear alone. Finally we find an average density profile consistent with an isothermal sphere.