Substructures in lens galaxies: PG1115+080 and B1555+375, two fold configurations

TL;DR

This paper investigates flux ratio anomalies in fold gravitational lens systems PG1115+080 and B1555+375, proposing substructures near the images as the cause, and models these with added singular isothermal spheres to match observations.

Contribution

It introduces a method to explain flux anomalies by adding substructures to smooth lens models, specifically analyzing two lens systems with new substructure parameters.

Findings

Substructures can account for flux ratio anomalies.

Models with substructures fit observed data well.

Mass estimates of substructures range from 10^5 to 10^8 solar masses.

Abstract

We study the anomalous flux ratio which is observed in some four-image lens systems, where the source lies close to a fold caustic. In this case two of the images are close to the critical curve and their flux ratio should be equal to unity, instead in several cases the observed value differs significantly. The most plausible solution is to invoke the presence of substructures, as for instance predicted by the Cold Dark Matter scenario, located near the two images. In particular, we analyze the two fold lens systems PG1115+080 and B1555+375, for which there are not yet satisfactory models which explain the observed anomalous flux ratios. We add to a smooth lens model, which reproduces well the positions of the images but not the anomalous fluxes, one or two substructures described as singular isothermal spheres. For PG1115+080 we consider a smooth model with the influence of the group of galaxies described by a SIS and a substructure with mass $\sim 10^{5} M_{\odot}$ as well as a smooth model with an external shear and one substructure with mass $\sim 10^{8} M_{\odot}$ . For B1555+375 either a strong external shear or two substructures with mass $\sim 10^{7} M_{\odot}$ reproduce the data quite well.