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. (arXiv:2003.12117v1 [astro-ph.GA])

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. (arXiv:2003.12117v1 [astro-ph.GA])
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In time-delay cosmography, three of the key ingredients are 1) determining
the velocity dispersion of the lensing galaxy, 2) identifying galaxies and
groups along the line of sight with sufficient proximity and mass to be
included in the mass model, and 3) estimating the external convergence
$kappa_mathrm{ext}$ from less massive structures that are not included in the
mass model. We present results on all three of these ingredients for two
time-delay lensed quasar systems, DES J0408-5354 and WGD 2038-4008. We use the
Gemini, Magellan and VLT telescopes to obtain spectra to both measure the
stellar velocity dispersions of the main lensing galaxies and to identify the
line-of-sight galaxies in these systems. Next, we identify 10 groups in DES
J0408-5354 and 2 groups in WGD 2038-4008using a group-finding algorithm. We
then identify the most significant galaxy and galaxy-group perturbers using the
“flexion shift” criterion. We determine the probability distribution function
of the external convergence $kappa_mathrm{ext}$ for both of these systems
based on our spectroscopy and on the DES-only multiband wide-field
observations. Using weighted galaxy counts, calibrated based on the Millennium
Simulation, we find that DES J0408-5354 is located in a significantly
underdense environment, leading to a tight (width $sim3%$), negative-value
$kappa_mathrm{ext}$ distribution. On the other hand, WGD 2038-4008 is located
in an environment of close to unit density, and its low source redshift results
in a much tighter $kappa_mathrm{ext}$ of $sim1%$, as long as no external
shear constraints are imposed.

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