http://arxiv.org/abs/1907.08638
Gravitational lensing offers a a competitive method to measure $H_0$ 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 $H_0$. The modeling process artificially selects one choice from this family, potentially biasing the recovered value of $H_0$. 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 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 over a range of radii. Constraining the slope to the true value near the image radius can introduce substantial bias (0-8% for our most realistic models). We conclude that lensing degeneracies manifest in a more complicated way than is assumed. If stellar kinematics incorrectly break the MSD, their inclusion may introduce more bias than their omission.
M. Gomer and L. Williams
Tue, 23 Jul 19
71/72
Comments: 13 pages, 9 figures