The stellar initial mass function and dark matter halo of ESO0286: Constraints from strong lensing and dynamics

Abstract

The internal mass structure of elliptical galaxies offers critical insights into galaxy formation, yet disentangling stellar mass from dark matter (DM) and determining the stellar initial mass function (IMF) remain challenging. We present a detailed analysis of ESO0286-G022 (z=0.0312), a rare nearby strong-lens system with a fast-rotating elliptical galaxy, combining high-resolution Hubble Space Telescope (HST) imaging with VLT/MUSE integral-field stellar kinematics. We constructed a series of axisymmetric and triaxial Schwarzschild orbit-superposition models to reconstruct its intrinsic shape and mass distribution. We find that despite being a fast rotator, ESO0286 exhibits clear kinematic signatures of intrinsic triaxiality, characterized by rotation along both the major and minor axes, making it only the second such confirmed case. Models based on kinematics alone yield significantly larger scatter in the predicted total mass at large radii. By incorporating the mass enclosed within the Einstein radius from strong lensing as a complementary constraint to the kinematic data, we were able to anchor the total mass at a radius where the constraints taken from the IFU data alone are weak. This allowed us to significantly reduce the uncertainty on the outer mass profile and orbital structure, establishing that only models with strong radial anisotropy beyond the IFU FoV are compatible with the lensing constraints. In the inner regions, we robustly constrained an upper limit for the stellar mass around r ∼ 0.7 kpc, ruling out an IMF more bottom-heavy than Kroupa. The data allowed for a gentle gradient toward a slightly more bottom-heavy central IMF. This result is well aligned with recent dynamical studies of local well-resolved massive early-type galaxies; however, it stands in contrast to the heavier IMFs reported for lenses detected at z > 0.1. Our work demonstrates the power of combining lensing and dynamical modeling to resolve the detailed inner structure of massive galaxies.