Gravitational lenses on arcseconds scales provide a tool to probe the mass
distribution in the lensing galaxies at redshift z ≈ 0.5–1.0. Image positions
can be fitted using simple smooth galaxy mass models, but observed fluxes are
more difficult to match. We analyze the effects of substructures in galaxy
halos to explain such anomalies in the cusp and fold lensing configurations
with different approaches.
In the first case we use detailed numerical simulations combined with a Monte Carlo approach
to compare predictions from the ΛCDM small scale mass function
with observed flux ratios. We extended our analysis down to a mass of ≈105M๏ for the subhalos. Moreover, we considered extra-halos, like other galaxies
surrounding the primary lens: also if we include these effects we are not able
to reproduce the observed fluxes.
This seems to indicate that there is no direct evidence for dark dwarf
satellites from multiple imaged QSOs.
In the second case we try to constrain, with a semianalytical approach, the mass
and the position of a substructure by considering its effects on the flux of
the images: we add to a smooth lens model, which reproduces well the positions
of the images but not the anomalous fluxes, one or two substructures described as singular isothermal spheres.
With substructures in the mass range ~ 106–108M๏
we are able to fit quite accurately the anomalous fluxes for fold configurations.