Systematics in ETG mass profile modelling: strong lensing & stellar dynamics

Abstract Strong gravitational lensing and stellar dynamics are independent and powerful methods to probe the total gravitational potential of galaxies, and thus, their total mass profile. However, inherent degeneracies in the individual models makes it difficult to obtain a full understanding of the distribution of baryons and dark matter (DM), although such degeneracies might be broken by the combination of these two tracers, leading to more reliable measurements of the mass distribution of the lens galaxy. We use mock data from IllustrisTNG50 to compare how dynamical-only, lens-only, and joint modelling can constrain the mass distribution of early-type galaxies (ETGs). The joint model consistently outperforms the other models, achieving a 2% accuracy in recovering the total mass within 2.5 R eff . The Einstein radius is robustly recovered for both lens-only and joint models, with the first showing a median fractional error of -5% and the latter a fractional error consistent with zero. The stellar mass-to-light ratio and total mass density slope are well recovered by all models. In particular, the dynamical-only model achieves an accuracy of 1% for the stellar mass-to-light ratio, while the accuracy of the mass density slope is typically of the order of 5% for all models. However, all models struggle to constrain integrated quantities involving DM and the halo parameters. Nevertheless, imposing more restrictive assumptions on the DM halo, such as fixing the scale radius, could alleviate some of the issues. Finally, we verify that the number of kinematical constraints (15, 35, 55 bins) on the kinematical map does not impact the models outcomes.