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 subhalos, 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 OSIRIS 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 [OIII] 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 fit 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% confidence interval for the perturber mass within 600
pc are: 8.2$^{+0.6}_{-0.8}$, 8.2$^{+0.6}_{-1}$ and 7.6$\pm0.3$
$\log_{10}$[M$_{\rm{sub}}$/M$_\odot$] respectively for a singular isothermal
sphere, a pseudo-Jaffe, and an NFW 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.
