TY - JOUR

T1 - Galaxy-lens determination of H0

T2 - constraining density slope in the context of the mass sheet degeneracy

AU - Gomer, M.

AU - Williams, L. L.R.

N1 - Publisher Copyright:
© 2020 IOP Publishing Ltd and Sissa Medialab.

PY - 2020/11

Y1 - 2020/11

N2 - Gravitational lensing offers a competitive method to measure H0 with the goal of 1% precision. A major obstacle comes in the form of lensing degeneracies, such as the mass sheet degeneracy (MSD), which make it possible for a family of density profiles to reproduce the same lensing observables but return different values of H0. The modeling process artificially selects one choice from this family, potentially biasing H0. The effect is more pronounced when the profile of a given lens is not perfectly described by the lens model, which will always be the case to some extent. To explore this, we quantify the bias and spread in H0 by creating quads from two-component mass models and fitting them with a power-law ellipse+shear model. We find that the bias does not correspond to the estimate one would calculate by transforming the profile into a power law near the image radius. We also emulate the effect of including stellar kinematics by performing fits where the slope is constrained to the true value. Informing the fit using the true value near the image radius can introduce substantial bias (0-23% depending on the model). We confirm using Jeans arguments that kinematic constraints can result in a biased value of H0, though the degree of bias depends on the region kinematic modeling probes in specific lenses. We conclude that lensing degeneracies manifest through commonplace modeling approaches in a more complicated way than is assumed in the literature. If stellar kinematics incorrectly break the MSD, their inclusion may introduce more bias than their omission.

AB - Gravitational lensing offers a competitive method to measure H0 with the goal of 1% precision. A major obstacle comes in the form of lensing degeneracies, such as the mass sheet degeneracy (MSD), which make it possible for a family of density profiles to reproduce the same lensing observables but return different values of H0. The modeling process artificially selects one choice from this family, potentially biasing H0. The effect is more pronounced when the profile of a given lens is not perfectly described by the lens model, which will always be the case to some extent. To explore this, we quantify the bias and spread in H0 by creating quads from two-component mass models and fitting them with a power-law ellipse+shear model. We find that the bias does not correspond to the estimate one would calculate by transforming the profile into a power law near the image radius. We also emulate the effect of including stellar kinematics by performing fits where the slope is constrained to the true value. Informing the fit using the true value near the image radius can introduce substantial bias (0-23% depending on the model). We confirm using Jeans arguments that kinematic constraints can result in a biased value of H0, though the degree of bias depends on the region kinematic modeling probes in specific lenses. We conclude that lensing degeneracies manifest through commonplace modeling approaches in a more complicated way than is assumed in the literature. If stellar kinematics incorrectly break the MSD, their inclusion may introduce more bias than their omission.

KW - galaxy dynamics

KW - gravitational lensing

UR - http://www.scopus.com/inward/record.url?scp=85096999854&partnerID=8YFLogxK

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U2 - 10.1088/1475-7516/2020/11/045

DO - 10.1088/1475-7516/2020/11/045

M3 - Article

AN - SCOPUS:85096999854

SN - 1475-7516

VL - 2020

JO - Journal of Cosmology and Astroparticle Physics

JF - Journal of Cosmology and Astroparticle Physics

IS - 11

M1 - 045

ER -