Detection of substructure with adaptive optics integral field spectroscopy of the gravitational lens B1422+231

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(-0.8)(+0.6), 8.2(-1)(+0.6) and 7.6 +/- 0.3 log(10)[M-sub/M-circle dot], 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.