We present models for the dark and luminous mass structure of 12 strong lensing early-type galaxies ( ETGs ) .
We combine pixel-based modelling of multiband HST/ACS imaging with Jeans modelling of kinematics obtained from Keck/ESI spectra to disentangle the dark and luminous contributions to the mass .
Assuming a gNFW profile for the dark matter halo and a spatially constant stellar-mass-to-light ratio \Upsilon _ { \star } for the baryonic mass , we infer distributions for \Upsilon _ { \star } consistent with IMFs that are heavier than the Milky Way ’ s ( with a global mean mismatch parameter relative to a Chabrier IMF \mu _ { \alpha c } = 1.80 \pm 0.14 ) and halo inner density slopes which span a large range but are generally cuspier than the dark-matter-only prediction ( \mu _ { \gamma ^ { \prime } } = 2.01 _ { -0.22 } ^ { +0.19 } ) .
We investigate possible reasons for overestimating the halo slope , including the neglect of spatially varying stellar-mas-to-light ratios and/or stellar orbital anisotropy , and find that a quarter of the systems prefer radially declining stellar-mass-to-light ratio gradients , but that the overall effect on our inference on the halo slope is small .
We suggest a coherent explanation of these results in the context of inside-out galaxy growth , and that the relative importance of different baryonic processes in shaping the dark halo may depend on halo environment .