Systematic biases from assuming co-aligned, equal-shape dark and stellar components

Ascertain whether the assumption that a lens galaxy’s dark matter halo and stellar component share the same axis ratio and position angle introduces additional systematic biases in strong-lensing analyses of elliptical galaxies used for time-delay cosmography.

Background

In constructing their deflector models, the authors assume that the dark matter halo and the stellar component share the same axis ratio and position angle. Although this assumption is common in prior work, misalignments and shape differences between light and dark components have been observed.

The authors test limited modifications (introducing an offset in position angle and making dark matter more spherical) and find negligible impact on H0 in their setup. However, they explicitly note that it remains unclear whether this simplifying assumption introduces additional systematic biases more generally.