Reducibility of intrinsic scatter in the internal mass-sheet parameter for SLACS-KCWI lenses

Determine whether the intrinsic scatter observed in the internal mass-sheet transform parameter λ_int among Sloan Lens ACS Survey (SLACS) deflectors, as inferred from Keck-KCWI spatially resolved stellar kinematics, can be reduced by adopting more sophisticated models (such as two-dimensional axisymmetric Jeans dynamical modeling and more advanced lens mass models), or whether this scatter is an irreducible property of the SLACS lens sample.

Background

In the TDCOSMO hierarchical framework, the internal mass-sheet transform parameter λ_int is introduced to conservatively capture deviations from a simple power-law mass profile while remaining directly degenerate with H0. The mass-sheet degeneracy is broken using stellar kinematics, whose precision and accuracy have been improved via high-SNR Keck-KCWI measurements for the SLACS sample.

When combining the time-delay lens sample (TDCOSMO-2025) with the SLACS external lens sample, the authors find that SLACS requires significant intrinsic scatter in λ_int, limiting the improvement in H0 precision that would otherwise be expected from the high-quality KCWI data. This raises the question of whether the observed scatter is due to modeling limitations that could be mitigated with more advanced dynamical and lens mass modeling, or whether the scatter reflects an inherent property of the SLACS sample.