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In time-delay cosmography, three of the key ingredients are 1) determiningthe velocity dispersion of the lensing galaxy, 2) identifying galaxies andgroups along the line of sight with sufficient proximity and mass to beincluded in the mass model, and 3) estimating the external convergence$\kappa_\mathrm{ext}$ from less massive structures that are not included in themass model. We present results on all three of these ingredients for twotime-delay lensed quasar systems, DES J0408-5354 and WGD 2038-4008. We use theGemini, Magellan and VLT telescopes to obtain spectra to both measure thestellar velocity dispersions of the main lensing galaxies and to identify theline-of-sight galaxies in these systems. Next, we identify 10 groups in DESJ0408-5354 and 2 groups in WGD 2038-4008using a group-finding algorithm. Wethen identify the most significant galaxy and galaxy-group perturbers using the"flexion shift" criterion. We determine the probability distribution functionof the external convergence $\kappa_\mathrm{ext}$ for both of these systemsbased on our spectroscopy and on the DES-only multiband wide-fieldobservations. Using weighted galaxy counts, calibrated based on the MillenniumSimulation, we find that DES J0408-5354 is located in a significantlyunderdense environment, leading to a tight (width $\sim3\%$), negative-value$\kappa_\mathrm{ext}$ distribution. On the other hand, WGD 2038-4008 is locatedin an environment of close to unit density, and its low source redshift resultsin a much tighter $\kappa_\mathrm{ext}$ of $\sim1\%$, as long as no externalshear constraints are imposed.

monthly notices of the royal astronomical society

STRIDES: Spectroscopic and photometric characterization of the  environment and effects of mass along the line of sight to the gravitational  lenses DES J0408-5354 and WGD 2038-4008