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[*] The velocity dispersion anisotropy and mass-to-light ratio of elliptical galaxies
van der Marel R.P.
MNRAS, 253, 710-726, 1991
© 1991. The Royal Astronomical Society. All Rights Reserved.

[*] Citations to this paper in the ADS

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 and Illingworth (1990) and van der Marel, Binney and Davies (1990). The projected kinematics are predicted from the observed surface photometry, assuming a constant mass-to-light ratio, and a velocity ellipsoid with sigma_r = sigma_theta (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 sigma_r > sigma_theta, 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) = cst x L^{0.35 +/- 0.05}. The quantity (v/sigma)* is identified as a second parameter in this relation. Galaxies with high (v/sigma)* 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 the (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 halos must be present, or more detailed triaxial models must be studied.


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Roeland van der Marel, marel@stsci.edu.
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