Improving the precision of time-delay cosmography with observations of galaxies along the line of sight

TL;DR

This paper presents a method to improve cosmological measurements from gravitational lensing by constraining line-of-sight mass effects using galaxy observations and simulations, reducing uncertainties significantly.

Contribution

The authors develop a new approach to constrain line-of-sight convergence using galaxy overdensity matching with mock catalogs, enhancing the precision of time-delay cosmography.

Findings

Uncertainty in line-of-sight convergence is reduced from 0.06 to 0.04 for overdense lines of sight.

For typical lines of sight, uncertainty is lowered to below 0.03, corresponding to less than 3% distance error.

Photometric redshifts from griK bands are nearly as effective as spectroscopic redshifts for constraining line-of-sight effects.

Abstract

In order to use strong gravitational lens time delays to measure precise and accurate cosmological parameters the effects of mass along the line of sight must be taken into account. We present a method to achieve this by constraining the probability distribution function of the effective line of sight convergence k_ext. The method is based on matching the observed overdensity in the weighted number of galaxies to that found in mock catalogs with k_ext obtained by ray-tracing through structure formation simulations. We explore weighting schemes based on projected distance, mass, luminosity, and redshift. This additional information reduces the uncertainty of k_ext from sigma_k $0.06 to ~0.04 for very overdense lines of sight like that of the system B1608+656. For more common lines of sight, sigma_k is reduced to ~<0.03, corresponding to an uncertainty of ~<3% on distance. This uncertainty has comparable effects on cosmological parameters to that arising from the mass model of the deflector and its immediate environment. Photometric redshifts based on g, r, i and K photometries are sufficient to constrain k_ext almost as well as with spectroscopic redshifts. As an illustration, we apply our method to the system B1608+656. Our most reliable k_ext estimator gives sigma_k=0.047 down from 0.065 using only galaxy counts. Although deeper multi-band observations of the field of B1608+656 are necessary to obtain a more precise estimate, we conclude that griK photometry, in addition to spectroscopy to characterize the immediate environment, is an effective way to increase the precision of time-delay cosmography.