Abstract
I use gravitational lensing to study in detail the distribution of matter in the Cosmic Horseshoe (SDSS J1148+1930), which consists of a nearly complete Einstein ring formed by a background galaxy at zs = 2.38115 that is highly magnified by a central lensing galaxy zl = 0.444. From HST images, I robustly identify 47 bright multiply-lensed knots in the ring as constraints for constructing parametric lens models for the central lensing galaxy. I find an elliptical power-law profile with density slope n = 1.71, along with up to fourth-order multiple perturbations for the lensing galaxy, and a minor external shear of γ = 0.039 best reproduces the observed image positions. A measure of the fidelity of our lens model is its ability to reproduce all of the multiply-lensed arcs that make up the ring which are not used as lensing constraints, and therefore serve as independent tests of our lens model. Although able to reproduce the majority of the bright knots, three knots near the critical curve - which are a factor of 2 - 4 brighter than their associated counterparts - stand out as being anomalously bright compared with the model prediction. By contrast, nearby knots in the same arcs have brightnesses approximately consistent with those predicted by our lens model. Our work demonstrates the need for substructures or granulations in dark matter that will be difficult to be explained away as deficiencies in smooth lens models, as is possible in lensed systems in which much fewer lensing constraints are available. Implementing granulations onto the inferred global mass profile of the lens, I demonstrate that ψDM can plausibly explain the brightness anomalies that our smooth lens model finds for the aforementioned knots in the Einstein ring.
Anyone interested is welcome to attend.