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 HST Frontier Fields program has allowed a significant improvement of mass and magnification measurements but lensing models still have a residual RMS between 0.2" and few arcseconds, 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, density 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 constraints (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 impact 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.

Measuring the Value of the Hubble Constant “à la Refsdal”

“…Depending on the complexity of the lens mass model and on the number of observed multiple images, Equation (3) shows that a ratio of ratios of angular-diameter distances can, in principle, be estimated, and from that the values of the relevant cosmological parameters inferred. This method can be employed effectively in lens galaxy clusters with a large number of spectroscopically confirmed multiple images, where different values of Ξ can be used at the same time, as recently illustrated by Caminha et al (2016) (see Johnson & Sharon 2016and Acebron et al 2017 for further discussion). We note that (1) time-delay distances are primarily sensitive to the value of H 0 , and more mildly on other cosmological parameters (see also Linder 2011); (2) a lensing system with several multiply imaged sources can provide constraints on the value of Ω m and Ω Λ (in ΛCDM), but it is insensitive to the value of H 0 (the value of the Hubble constant cancels out in the ratio of Equation (3)).…”

Strong Lens Models for 37 Clusters of Galaxies from the SDSS Giant Arcs Survey*

“…We used the Gapper estimator for clusters with Nspec < 15 and the bi- We estimate the statistical uncertainties on individual model parameters from the MCMC sampling of the parameter space. These uncertainties do not include systematic uncertainties due to modeling choices, structure along the line of sight, or correlated structure and substructure, which have been shown to cause systematic uncertainties (e.g., Acebron et al 2017;Meneghetti et al 2017;Zitrin et al 2015;Cerny et al 2018;Priewe et al 2017;Johnson et al 2017a). From the lensing potential we compute the mass distribution, convergence, shear, magnification, and deflection maps.…”

On the Observability of Individual Population III Stars and Their Stellar-mass Black Hole Accretion Disks through Cluster Caustic Transits

“…Estimates of Cluster Caustic Transits for Pop III stars: To estimate the caustic transit rate and duration for Pop III stars, we first need to evaluate the plausible limits to the transverse velocities of massive lensing clusters, their typical caustic lengths, and the possible effects from microlensing. A Pop III caustictransit observing program with JWST should select the best lensing clusters with matching prior HST/ACS and WFC3 images, such as the Hubble Frontier Field clusters (HFF; e.g., Lotz et al 2017;Kawamata et al 2016;Lagattuta et al 2017;Acebron et al 2017;Mahler et al 2018) or the CLASH clusters (e.g., Postman et al 2012;Rydberg et al 2015). Given the significant differences in the allowed v T -values between the three HFF clusters discussed in W18, we adopt an upper limit of V T , s < ∼ 1000 km s −1 .…”