Systematic variations of central mass density slopes in early-type galaxies

We study the total density distribution in the central regions (less than or similar to 1 effective radius, R-e) of early-type galaxies (ETGs), using data from SPIDER and ATLAS(3D). Our analysis extends the range of galaxy stellar mass (M-star) probed by gravitational lensing, down to similar to 10(10)M(circle dot). We model each galaxy with two components (dark matter halo + stars), exploring different assumptions for the dark matter halo profile (i. e. NFW, NFW-contracted, and Burkert profiles), and leaving stellar mass-to-light (M-star/L) ratios as free fitting parameters to the data. For all plausible halo models, the best-fitting M-star/L, normalized to that for a Chabrier initial mass function, increases systematically with galaxy size and mass. For anNFWprofile, the slope of the total mass profile is non-universal, independently of several ingredients in the modelling (e. g. halo contraction, anisotropy, and rotation velocity in ETGs). For the most massive (M-star similar to 10(11.5)M(circle dot)) or largest (R-e similar to 15 kpc) ETGs, the profile is isothermal in the central regions (similar to R-e/2), while for the low-mass (M-star similar to 10(10.2)M(circle dot)) or smallest (R-e similar to 0.5 kpc) systems, the profile is steeper than isothermal, with slopes similar to those for a constant-M/L profile. For a steeper concentration-mass relation than that expected from simulations, the correlation of density slope with galaxy mass tends to flatten, while correlations with Re and velocity dispersions are more robust. Our results clearly point to a 'non-homology' in the total mass distribution of ETGs, which simulations of galaxy formation suggest may be related to a varying role of dissipation with galaxy mass.