Mass distribution and orbital anisotropy of early-type galaxies: constraints from the Mass Plane
Abstract
Massive early-type galaxies are observed to lie on the Mass Plane (MP), a two-dimensional manifold in the space of effective radius Re, projected mass Mp (measured via strong gravitational lensing) and projected velocity dispersion sigma within Re/2. The MP is less `tilted' than the Fundamental Plane, and the two have comparable associated scatter. This means that ce2=2*G*Mp/(Re*sigma2) is a nearly universal constant in the range sigma=175-400 km/s. This finding can be used to constrain the mass distribution and internal dynamics of early-type galaxies. We find that a relatively wide class of spherical galaxy models has values of ce2 in the observed range, because ce2 is not very strongly sensitive to the mass distribution and orbital anisotropy. If the total mass distribution is isothermal, a broad range of stellar luminosity profile and anisotropy is consistent with the observations, while NFW dark-matter halos require more fine tuning of the stellar mass fraction, luminosity profile and anisotropy. If future data can cover a broader range of masses, the MP could be seen to be tilted and the value of any such tilt would provide a discriminant between models for the total mass-density profile of the galaxies. [Abridged]