Open AccessMass profiles and anisotropies of early‐type galaxies
Author(s) -
Magorrian John,
Ballantyne David
Publication year - 2001
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2001.04150.x
Subject(s) - physics , astrophysics , galaxy , elliptical galaxy , astronomy , radial velocity , disc galaxy , anisotropy , stellar mass , luminosity , peculiar galaxy , stars , star formation , quantum mechanics
We discuss the problem of using stellar kinematics of early‐type galaxies to constrain the orbital anisotropies and radial mass profiles of galaxies. We demonstrate that compressing the light distribution of a galaxy along the line of sight produces approximately the same signature in the line‐of‐sight velocity profiles as radial anisotropy. In particular, fitting spherically symmetric dynamical models to apparently round, isotropic face‐on flattened galaxies leads to a spurious bias towards radial orbits in the models, especially if the galaxy has a weak face‐on stellar disc. Such face‐on stellar discs could plausibly be the cause of the radial anisotropy found in spherical models of intermediate luminosity ellipticals such as NGC 2434, 3379 and 6703. In the light of this result, we use simple dynamical models to constrain the outer mass profiles of a sample of 18 round, early‐type galaxies. The galaxies follow a Tully–Fisher relation parallel to that for spiral galaxies, but fainter by at least 0.8 mag ( I ‐band) for a given mass. The most luminous galaxies show clear evidence for the presence of a massive dark halo, but the case for dark haloes in fainter galaxies is more ambiguous. We discuss the observations that would be required to resolve this ambiguity.