The Impact of Orbital Anisotropy Assumptions in Lensing-dynamics Modeling

Abstract We investigate potential systematic biases introduced by assumptions regarding stellar orbital anisotropy in joint lensing-dynamics modeling. Using massive early-type galaxies from the TNG100 simulation at redshifts z = 0.2, 0.5, and 0.7, we generate a self-consistent mock data set containing lensing and stellar kinematic observables. This is achieved through taking the potential from both dark matter and baryons of the galaxies, plus the radial variation of the orbit anisotropy depicted by a logistic function. By integrating constraints from strong lensing, weak lensing, and stellar kinematics, we disentangle the contributions of stars and dark matter inside the galaxies. Under commonly adopted stellar anisotropy assumptions (isotropic orbits, constant anisotropy, and the Osipkov–Merritt profile), we find no significant systematic bias in key inferred parameters at the population level: the stellar mass bias Δ log M ⋆ / M ⊙ = − 0.03 ± 0.10 dex , the dark matter fraction bias Δ f dm = 2% ± 10%, and the dark matter inner density slope bias Δ η dm = 0.15 ± 0.2. In other words, the inferences on stellar mass, dark matter fraction, and on the inner dark matter slopes are consistent with no bias within a 1 σ error. Additionally, this lack of significant biases is insensitive to the discrepancies between the assumed anisotropy in modeling and the real orbital anisotropy of the mock sample. Our results suggest that conventional assumptions regarding stellar orbital anisotropy do not introduce a significant bias when inferring galaxy mass density distribution at the population level.