Title: Hubble Frontier Fields: systematic errors in strong lensing models of galaxy clusters - Implications for cosmography,
Abstract: Strong gravitational lensing by galaxy clusters is a fundamental tool to
study dark matter and constrain the geometry of the Universe. Recently, the
Hubble Space Telescope Frontier Fields programme has allowed a significant
improvement of mass and magnification measurements but lensing models still
have a residual root mean square between 0.2 arcsec and few arcsec- onds, not
yet completely understood. Systematic errors have to be better understood and
treated in order to use strong lensing clusters as reliable cosmological
probes. We have analysed two simulated Hubble-Frontier-Fields-like clusters
from the Hubble Frontier Fields Comparison Challenge, Ares and Hera. We use
several estimators (relative bias on magnification, den- sity profiles,
ellipticity and orientation) to quantify the goodness of our reconstructions by
comparing our multiple models, optimized with the parametric software LENSTOOL
, with the input models. We have quantified the impact of systematic errors
arising, first, from the choice of different density profiles and
configurations and, secondly, from the availability of con- straints
(spectroscopic or photometric redshifts, redshift ranges of the background
sources) in the parametric modelling of strong lensing galaxy clusters and
therefore on the retrieval of cosmological parameters. We find that
substructures in the outskirts have a significant im- pact on the position of
the multiple images, yielding tighter cosmological contours. The need for
wide-field imaging around massive clusters is thus reinforced. We show that
competitive cosmological constraints can be obtained also with complex
multimodal clusters and that photometric redshifts improve the constraints on
cosmological parameters when considering a narrow range of (spectroscopic)
redshifts for the sources.