The outer haloes of massive, elliptical galaxies

The outer haloes of massive elliptical galaxies are dark-matter dominated regions where stellar orbits have longer dynamical timescales than the central regions and therefore better preserve their formation history. Dynamical models out to large radii suffer from a degeneracy between mass and orbital structure, as the outer kinematics are unable to resolve higher moments of the line-of-sight velocity distribution. We mitigate this degeneracy for a sample of quiescent, massive, nearby ellipticals by determining their mass distributions independently using a non-parametric method on X-ray observations of the surrounding hot interstellar medium. We then create dynamical models using photometric and kinematic constraints consisting of integral-eld, long-slit and planetary nebulae (PNe) data extending to 50 kpc. The rst two galaxies of our sample, NGC 5846 and NGC 1399, were found to have very shallow projected light distributions with a power law index of 1.5 and a dark matter content of 70-80% at 50 kpc. Spherical Jeans models of the data show that, in the outer haloes of both galaxies, the projected velocity dispersions are almost inde- pendent of the anisotropy and that the PNe prefer the lower end of the range of mass distributions consistent with the X-ray data. Using the N-body code NMAGIC, we cre- ated axisymmetric models of NGC 5846 using the individual PNe radial velocities in a likelihood method and found them to be more constraining than the binned velocity dispersions. Characterising the orbital structure in terms of spherically averaged proles of the velocity dispersions we nd σ_ψ \gt σ_r \gt σ_θ.

The outer haloes of massive, elliptical galaxies. Highlights of Astronomy , 15 92-92 - 92-92. 10.1017/S1743921310008446. 

Das, P; Gerhard, O; de Lorenzi, F; McNeil, E; Churazov, E; Coccato, L; (2010)