- [*] The velocity dispersion anisotropy of elliptical galaxies
- van der Marel R.P.
- in `Structure, dynamics, and chemical evolution of elliptical
galaxies', Danziger I.J., Zeilinger W.W., Kjar K., eds.,
ESO, Garching, Germany, p. 79-86, 1993
- © 1993. European Southern Observatory.
All Rights Reserved.
It has been known for some time that elliptical galaxies are not
flattened by rotation but by the anisotropy of their velocity
dispersion tensor. However, it is not known what the sense of this
anisotropy is, ``radial'' or ``tangential''. For flattened galaxies a
constraint on the velocity dispersion anisotropy can be obtained by
comparing the predictions of dynamical models to kinematical
observations on both the major and the minor axis. Models in which the
distribution function depends on only the two classical integrals of
motion, f=f(E,L_z), generally predict too much motion on the major
axis relative to the minor axis (van der Marel
1991). This implies that real elliptical galaxies must have more
radial motion than these models, i.e., must have sigma_r >
sigma_theta. A new and promising method to further constrain the
velocity dispersion anisotropy of elliptical galaxies relies on
measuring symmetric deviations of observed line profiles from
Gaussians. This is now possible. Line profiles are discussed for a
simple set of models with different velocity dispersion
anisotropy. Fitting Gaussians to such line profiles can introduce
systematic errors in the estimates of rotation velocities and velocity
dispersions. A useful method to quantify deviations of observed line
profiles from Gaussians relies on expanding the line profile in a
Gauss-Hermite series, as was recently argued by van der Marel & Franx (1992) and independently
by Gerhard (this volume; 1992).
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Last modified December 8, 1998.
Roeland van der Marel,