  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.

Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies