Dark matter contraction and stellar-mass-to-light ratio gradients in massive early-type galaxies

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 ($\mu_{\gamma'} = 2.01_{-0.22}^{+0.19}$). 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.