Mass distribution and orbital anisotropy of early-type galaxies: constraints from the Mass Plane

TL;DR

This study investigates the mass distribution and orbital anisotropy of early-type galaxies using the Mass Plane, revealing a nearly universal constant that constrains galaxy models and suggests future data could distinguish between different mass-density profiles.

Contribution

The paper demonstrates that the Mass Plane constrains galaxy mass distribution and dynamics, showing that a wide range of models fit current observations and proposing future data to differentiate models.

Findings

c_e2 is nearly universal across observed galaxies.

Isothermal mass distribution models are broadly consistent with data.

NFW halos require fine-tuning of parameters.

Abstract

Massive early-type galaxies are observed to lie on the Mass Plane (MP), a two-dimensional manifold in the space of effective radius R_e, projected mass M_p (measured via strong gravitational lensing) and projected velocity dispersion sigma within R_e/2. The MP is less `tilted' than the Fundamental Plane, and the two have comparable associated scatter. This means that c_e2=2*G*M_p/(R_e*sigma^2) 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 c_e2 in the observed range, because c_e2 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]