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 $10.2<\log M_\ast/M_\odot<11.7$). We
combine observations from the SLUGGS and Atlas3D 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, and employ quite general
profiles for the dark halos, and in particular do not place any restriction 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
$\rho_{\rm tot}(r)\propto r^{-\gamma}$ from $R_e$/10 to at least 4$R_e$, the
largest average deviation being 11%. The average logarithmic slope is
$\langle\gamma\rangle=2.19\pm0.03$ with observed rms scatter of just
$\sigma_\gamma=0.11$. This scatter out to large radii, where dark matter
dominates, is as small as previously reported by lensing studies around
$r\approx R_e/2$, 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 would now
be important to test the generality of our results for different galaxy types
and larger samples.
