Quantifying Environmental and Line-of-sight Effects in Models of Strong Gravitational Lens Systems

Abstract

Matter near a gravitational lens galaxy or projected along the line of sight (LOS) can affect strong lensing observables by more than contemporary measurement errors. We simulate lens fields with realistic threedimensional mass configurations (self-consistently including voids), and then fit mock lensing observables with increasingly complex lens models to quantify biases and uncertainties associated with different ways of treating the lens environment (ENV) and LOS. We identify the combination of mass, projected offset, and redshift that determines the importance of a perturbing galaxy for lensing. Foreground structures have a stronger effect on the lens potential than background structures, due to nonlinear effects in the foreground and downweighting in the background. There is dramatic variation in the net strength of ENV/LOS effects across different lens fields; modeling fields individually yields stronger priors for H 0 than ray tracing through N-body simulations. Models that ignore mass outside the lens yield poor fits and biased results. Adding external shear can account for tidal stretching from galaxies at redshifts  z z lens , but it requires corrections for external convergence and cannot reproduce nonlinear effects from foreground galaxies. Using the tidal approximation is reasonable for most perturbers as long as nonlinear redshift effects are included. Even then, the scatter in H 0 is limited by the lens profile degeneracy. Asymmetric image configurations produced by highly elliptical lens galaxies are less sensitive to the lens profile degeneracy, so they offer appealing targets for precision lensing analyses in future surveys like LSST and Euclid.

Modelling the line-of-sight contribution in substructure lensing

“…As shown by McCully et al (2017), since the deflection angle of the foreground lens enters the argument of the deflection angle of the background lens, non-linear lensing effects are introduced when the mass of the former is large enough. However, the masses of our perturbers are much smaller than the mass of the main lens, by 3 to 7 orders of magnitude, and thus when the perturber is in the foreground its effect on the main lens is small, while the opposite holds when the perturber is in the background and its deflection angle is influenced by the presence of the main lens.…”

“…Birrer et al 2016;McCully et al 2017). At sub-galactic scales, a significant effort has E-mail:gdespali@mpa-garching.mpg.de been made over the years to understand the line-of-sight contribution to the flux-ratio anomalies observed in gravitationally lensed quasars (e.g.…”

The influence of weak lensing on measurements of the Hubble constant with quad-image gravitational lenses

“…Following McCully et al (2016) we assume that the image positions are measured with the accuracy of 0.003 arcsec, the lens position to 0.003 arcsec, the flux ratios to 0.05 mag, and the time delays to 1 d. The redshifts of the lens and source are fixed and we do not consider the influence of their accuracy on the results.…”

“…The problem of the environment influence is investigated in their work by Suyu et The description of light travel in a nonuniform universe model, which addresses part of the problems arising in connection with H 0 derivation, is still being improved. McCully et al (2014), McCully et al (2016) using a multiplane approach, assume that in most layers the deflection can be modeled as shear, but they allow for more than a single layer, where the full treatment can be applied. This approach saves computation time by using shear approximation in majority of layers but its algebra is rather complicated.…”

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses

“…Such models, in combination with an estimate of the overall mass along the line of sight (e.g. Hilbert et al 2009;McCully et al 2017McCully et al , 2014Collett et al 2013) allow one to measure the time-delay distance and consequently the Hubble parameter, H 0 (e.g. Suyu et al 2010).…”