What can strong lensing recover in edge-on disc galaxies? A mock study based on TNG50 simulations

Abstract

Strong gravitational lensing tightly constrains the projected mass within the Einstein aperture, but what else imaging alone can recover for edge-on disc lenses remains untested against known truth. We construct HST/ACS-like mock observations of 35 edge-on TNG50 disc galaxies selected to resemble lenses from the Sloan WFC Edge-on Late-type Lens Survey (SWELLS), and fit them with a five-stage Source-Light-and-Mass workflow in PyAutoLens; the final model represents the lens mass distribution by scaling the Multi-Gaussian Expansion (MGE) foreground-light template with a global mass-to-light ratio and adding a spherical NFW halo. Decomposed fits succeed for 21 of the 35 systems; in the other 14, coherent MGE lens-light residuals prevent reliable modelling. For the successful subset, the Einstein radius and the total aperture mass are recovered with normalised median absolute deviation (NMAD) scatters of 0.3% and 0.7%. The global projected mass-weighted slope has an NMAD scatter of 10.3%, smaller than that of the mass-sheet-invariant reduced deflection curvature $ξ$ (11.4%) and well below that of the local slope (40.7%), while the stellar and dark-matter aperture masses scatter by 15.1% and 108.8%. A 35-system true-light diagnostic using the exact input light gives component-mass scatters of 0.8% and 6.3%. Under these controlled assumptions, imaging constrains Einstein-scale total mass and global radial structure more reliably than local radial structure or the stellar-dark-matter decomposition.