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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 (AGNs) are particularly sensitive to perturbing subhaloes, but their use as a test of CDM has been limited by the small number of systems which have significant radio emission that is extended enough to 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 Wide Field Camera 3 (WFC3) grism narrow-line survey of lensed quasars, for the quadruply lensed AGN HE 0435–1223. Using a forward modelling pipeline that enables us to robustly account for spatial blending, we measure the [O III] 5007 Å flux ratios of the four images. We find that the [O III] fluxes and positions are well fit by a simple smooth mass model for the main lens. Our data rule out a M600 > 108(107.2)M⊙ Navarro–Frenk–White perturber projected within ~1.0 (0.1) arcsec of each of the lensed images, where M600 is the perturber mass within its central 600 pc. The non-detection is broadly consistent with the expectations of ΛCDM for a single system. The sensitivity achieved demonstrates that powerful limits on the nature of dark matter can be obtained with the analysis of ~20 narrow-line lenses. [ABSTRACT FROM AUTHOR]

Strong gravitational lenses can be used to detect low-mass subhaloes, based on deviations in image fluxes and positions from what can be achieved with a smooth mass distribution. So far, this method has been limited by the small number of (radio-loud, microlensing-free) systems which can be analysed for the presence of substructure. Using the gravitational lens B1422+231, we demonstrate that adaptive optics integral field spectroscopy can also be used to detect dark substructures. We analyse data obtained with OH Suppressing Infra-Red Imaging Spectrograph on the Keck i Telescope, using a Bayesian method that accounts for uncertainties relating to the point spread function and image positions in the separate exposures. The narrow-line [O iii] fluxes measured for the lensed images are consistent with those measured in the radio, and show a significant deviation from what would be expected in a smooth mass distribution, consistent with the presence of a perturbing low-mass halo. Detailed lens modelling shows that image fluxes and positions are fitted significantly better when the lens is modelled as a system containing a single perturbing subhalo in addition to the main halo, rather than by the main halo on its own, indicating the significant detection of substructure. The inferred mass of the subhalo depends on the subhalo mass density profile: the 68 per cent confidence intervals for the perturber mass within 600 pc are 8.2$^{+0.6}_{-0.8}$, 8.2$^{+0.6}_{-1}$ and 7.6 ± 0.3 log10[Msub/M⊙], respectively, for a singular isothermal sphere, a pseudo-Jaffe and a Navarro–Frenk–White mass profile. This method can extend the study of flux ratio anomalies to virtually all quadruply imaged quasars, and therefore offers great potential to improve the determination of the subhalo mass function in the near future. [ABSTRACT FROM AUTHOR]