Zooming into the Cosmic Horseshoe: new insights on the lens profile and the source shape

TL;DR

This study uses detailed lens modeling of the Cosmic Horseshoe to refine the dark matter profile, revealing a flatter-than-isothermal slope and discussing implications for structure formation theories.

Contribution

It introduces a method to break degeneracies in lens slope and source size, providing new constraints on the dark matter distribution in a prominent Einstein ring.

Findings

Dark matter slope is flatter than isothermal, with t ≈ 0.08.

Separating baryonic matter alters the dark matter slope estimate to t ≈ 0.31 or 0.44.

Results align with predictions from LambdaCDM structure formation models.

Abstract

The gravitational lens SDSS J1148+1930, also known as the Cosmic Horseshoe, is one of the biggest and of the most detailed Einstein rings ever observed. We use the forward reconstruction method implemented in the lens fitting code Lensed to investigate with great detail the properties of the lens and of the background source. We model the lens with different mass distributions, focusing in particular on the determination of the slope of the dark matter component. The inherent degeneracy between the lens slope and the source size can be broken when we can isolate separate components of each lensed image, as in this case. For an elliptical power law model, $\kappa(r) \sim r^{-t}$, the results favour a flatter-than-isothermal slope with a maximum-likelihood value t = 0.08. Instead, when we consider the contribution of the baryonic matter separately, the maximum-likelihood value of the slope of the dark matter component is t = 0.31 or t = 0.44, depending on the assumed Initial Mass Function. We discuss the origin of this result by analysing in detail how the images and the sources change when the slope t changes. We also demonstrate that these slope values at the Einstein radius are not inconsistent with recent forecast from the theory of structure formation in the LambdaCDM model.