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2.1 The dynamics of early-type galaxies
Since nearly 30 years it is
known that spiral galaxies have flat rotation curves and therefore
dark matter halos, but only recently it has been possible to establish
methods based on stellar dynamics to assess the presence of dark halos
around the generic (i.e., not restricted to objects with X-ray halos
or gaseous disks) elliptical Measuring not only the stellar projected
mean velocities and velocity dispersions, but also the line-of-sight
velocity distributions from deep spectroscopic observations it is
possible to measure the anisotropy of the velocity distribution
(ellipticals appear nearly isotropic or slightly radially anisotropic)
and therefore the gravitational potential of an elliptical galaxy
(ellipticals do have flat rotation curves like spirals). The analysis
of a set of 20 round non-rotating ellipticals indicates that the dark
halos of ellipticals are a factor 25 denser than the ones of spirals,
implying a high redshift of formation (z ~ 5) or the presence
of a baryonic component. We are extending this work to flattened
galaxies using the Schwarzschild orbit superposition method.
last update:
10/2004, editor of this page: Roberto Philip Saglia
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© Max-Planck-Institut für extraterrestrische Physik
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