Can we use Weak Lensing to Measure Total Mass Profiles of Galaxies on 20 kiloparsec Scales?

TL;DR

This paper investigates the potential of using weak lensing on scales of 20-40 kpc to measure galaxy mass profiles, focusing on the inner regions where stellar and dark matter components are comparable, with forecasts for upcoming surveys.

Contribution

It introduces a method to probe galaxy inner mass profiles using weak lensing at small scales and assesses the feasibility with future surveys like Euclid and WFIRST.

Findings

Euclid and WFIRST can constrain profiles at Req with SNR >20 for galaxies >10^{10} solar masses.

Rejection of close source galaxies reduces source density by 20%.

Weak lensing at small scales combined with stellar kinematics can constrain dark matter profiles.

Abstract

Current constraints on dark matter density profiles from weak lensing are typically limited to radial scales greater than 50-100 kpc. In this paper, we explore the possibility of probing the very inner regions of galaxy/halo density profiles by measuring stacked weak lensing on scales of only a few tens of kpc. Our forecasts focus on scales smaller than the equality radius (Req) where the stellar component and the dark matter component contribute equally to the lensing signal. We compute the evolution of Req as a function of lens stellar mass and redshift and show that Req=7-34 kpc for galaxies with the stellar mass of 10^{9.5}-10^{11.5} solar masses. Unbiased shear measurements will be challenging on these scales. We introduce a simple metric to quantify how many source galaxies overlap with their neighbours and for which shear measurements will be challenging. Rejecting source galaxies with close-by companions results in about a 20 per cent decrease in the overall source density. Despite this decrease, we show that Euclid and WFIRST will be able to constrain galaxy/halo density profiles at Req with signal-to-noise ratio >20 for the stellar mass of >10^{10} solar masses. Weak lensing measurements at Req, in combination with stellar kinematics on smaller scales, will be a powerful means by which to constrain both the inner slope of the dark matter density profile as well as the mass and redshift dependence of the stellar initial mass function.