The velocity dispersion anisotropy and mass-to-light ratio of elliptical galaxies.
Abstract
Axisymmetric dynamical models are constructed for 37 bright elliptical galaxies for which high-quality photometrical and both major and minor axis kinematical data are available in the literature. The models are of the type used previously by Binney, Davies & Illingworth and van der Marel, Binney & Davies. The projected kinematics are predicted from the observed surface photometry, assuming a constant mass-to-light ratio, and a velocity ellipsoid with σ_r_ = σtheta_ [i.e. f=f(E,L_Z_)]. For the sample as a whole it is found that these models tend to predict too much motion on the major axis. This implies that elliptical galaxies as a class must have σ_r_ > σtheta_ which has not been demonstrated before. This result is consistent with general expectation as based on N-body simulations of dissipationless collapse. From the models accurate mass-to-light ratios are derived that are corrected for the effects of rotation and radial anisotropy. The average mass-to-light ratio for the galaxies in the sample is (M/L)_B_ = (5.93 +/- 0.25)h_50_. The mass-to-light ratios correlate with total luminosity according to (M/L) is proportional to L^0.35+/-0.05^. The quantity (v/σ)^*^ is identified as a second parameter in this relation. Galaxies with high (v/σ)^*^ tend to have a low mass-to- light ratio (for their luminosity). The observed velocities in the outer parts of the galaxies in the sample systematically exceed the velocities predicted by our (constant mass-to-light ratio) models. From this it is argued that no axisymmetric constant mass-to-light ratio models can fit the kinematical data in the outer parts of elliptical galaxies. Either dark haloes must be present, or more detailed triaxial models must be studied.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- December 1991
- DOI:
- 10.1093/mnras/253.4.710
- Bibcode:
- 1991MNRAS.253..710V
- Keywords:
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- Elliptical Galaxies;
- Mass To Light Ratios;
- Velocity Distribution;
- Anisotropy;
- Astronomical Models;
- Astronomical Photometry;
- Dynamic Models;
- Galactic Structure;
- Astrophysics