Small Scatter and Nearly Isothermal Mass Profiles to Four Half-light Radii from Two-dimensional Stellar Dynamics of Early-type Galaxies

We study the total mass-density profile for a sample of 14 fast-rotator early-type galaxies (stellar masses 10.2≲ log {{M}_*}/{{M}_⊙ }≲ 11.7). We combine observations from the SLUGGS and ATLAS^3D surveys to map out the stellar kinematics in two dimensions, out to a median radius for the sample of four half-light radii R_e (or 10 kpc) and a maximum radius of 2.0-6.2 R_e (or 4-21 kpc). We use axisymmetric dynamical models based on the Jeans equations, which allow for a spatially varying anisotropy; employ quite general profiles for the dark halos; and, in particular, do not place any restrictions on the profile slope. This is made possible by the availability of spatially extended two-dimensional kinematics. We find that our relatively simple models provide a remarkably good description of the observed kinematics. The resulting total density profiles are well described by a nearly isothermal power law {{ρ }_tot}(r)\propto {{r}^-γ } from R_e/10 to at least 4R_e, the largest average deviation being 11%. The average logarithmic slope is < γ > =2.19+/- 0.03 with observed rms scatter of just {{σ }_γ }=0.11. This scatter out to large radii, where dark matter dominates, is as small as previously reported by lensing studies around r ≈ R_e/2, where the stars dominate. Our bulge-halo conspiracy places much tighter constraints on galaxy formation models. It illustrates the power of two-dimensional stellar kinematics observations at large radii. It is now important to test the generality of our results for different galaxy types and larger samples.