# Probing dark matter substructure in the gravitational lens HE0435-1223 with the WFC3 grism

## TL;DR

This study uses WFC3 grism observations of the quadruply lensed quasar HE0435-1223 to place constraints on dark matter substructure, demonstrating the potential of narrow-line flux ratios to test Cold Dark Matter models.

## Contribution

First application of WFC3 grism narrow-line imaging to probe dark matter substructure in a lensed quasar, providing a new method to detect or rule out low-mass halos.

## Key findings

- No significant dark matter subhalos with M_{600} > 10^{8} M_ ext{sun} detected near the images.
- Results are consistent with ΛCDM predictions for a single lens system.
- Method demonstrates high sensitivity, promising for future dark matter studies with ~20 lenses.

## Abstract

Strong gravitational lensing provides a powerful test of Cold Dark Matter (CDM) as it enables the detection and mass measurement of low mass haloes even if they do not contain baryons. Compact lensed sources such as Active Galactic Nuclei (AGN) are particularly sensitive to perturbing subhalos, but their use as a test of CDM has been limited by the small number of systems which have significant radio emission which is extended enough avoid significant lensing by stars in the plane of the lens galaxy, and red enough to be minimally affected by differential dust extinction. Narrow-line emission is a promising alternative as it is also extended and, unlike radio, detectable in virtually all optically selected AGN lenses. We present first results from a WFC3 grism narrow-line survey of lensed quasars, for the quadruply lensed AGN HE0435-1223. Using a forward modelling pipeline which enables us to robustly account for spatial blending, we measure the [OIII] 5007 \AA~ flux ratios of the four images. We find that the [OIII] fluxes and positions are well fit by a simple smooth mass model for the main lens. Our data rule out a $M_{600}>10^{8} (10^{7.2}) M_\odot$ NFW perturber projected within $\sim$1\farcs0 (0\farcs1) arcseconds of each of the lensed images, where $M_{600}$ is the perturber mass within its central 600 pc. The non-detection is broadly consistent with the expectations of $\Lambda$CDM for a single system. The sensitivity achieved demonstrates that powerful limits on the nature of dark matter can be obtained with the analysis of $\sim20$ narrow-line lenses.