Direct Shear Mapping - a new weak lensing tool

TL;DR

Direct Shear Mapping (DSM) is a novel weak lensing technique that measures gravitational shear directly from the velocity maps of individual galaxies, utilizing their rotational symmetry and velocity distortions.

Contribution

This paper introduces DSM, a new method that directly infers gravitational shear from single galaxy velocity maps, contrasting with traditional ensemble averaging approaches.

Findings

Able to measure shears as small as 0.01

Null results obtained for un-lensed low-redshift galaxies with ±0.01 error

Utilizes spatially resolved spectroscopic data including velocity information

Abstract

We have developed a new technique called Direct Shear Mapping (DSM) to measure gravitational lensing shear directly from observations of a single background source. The technique assumes the velocity map of an un-lensed, stably-rotating galaxy will be rotationally symmetric. Lensing distorts the velocity map making it asymmetric. The degree of lensing can be inferred by determining the transformation required to restore axisymmetry. This technique is in contrast to traditional weak lensing methods, which require averaging an ensemble of background galaxy ellipticity measurements, to obtain a single shear measurement. We have tested the efficacy of our fitting algorithm with a suite of systematic tests on simulated data. We demonstrate that we are in principle able to measure shears as small as 0.01. In practice, we have fitted for the shear in very low redshift (and hence un-lensed) velocity maps, and have obtained null result with an error of $\pm 0.01$. This high sensitivity results from analysing spatially resolved spectroscopic images (i.e. 3D data cubes), including not just shape information (as in traditional weak lensing measurements) but velocity information as well. Spirals and rotating ellipticals are ideal targets for this new technique. Data from any large IFU or radio telescope is suitable, or indeed any instrument with spatially resolved spectroscopy such as SAMI, ALMA, HETDEX and SKA.