Issue |
A&A
Volume 429, Number 1, January I 2005
|
|
---|---|---|
Page(s) | 161 - 172 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20041023 | |
Published online | 13 December 2004 |
A family of models of partially relaxed stellar systems
I. Dynamical properties
1
Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy e-mail: m.trenti@sns.it
2
Dipartimento di Fisica, Università di Milano, via Celoria 16, 20133 Milano, Italy e-mail: Giuseppe.Bertin@unimi.it
Received:
5
April
2004
Accepted:
27
August
2004
Recently we have found that a family of models of
partially relaxed, anisotropic stellar systems, inspired earlier by
studies of incomplete violent relaxation, exhibits some interesting
thermodynamic properties. Here we present a systematic investigation
of its dynamical characteristics, in order to establish the basis for
a detailed comparison with simulations of collisionless collapse,
planned for a separate paper. For a full comparison with the
observations of elliptical galaxies, the models should be extended to
allow for the presence a sizable dark halo and of significant
rotation. In the spherical limit, the family is characterized by two
dimensionless parameters, i.e. Ψ, measuring the depth of the
galaxy potential, and ν, defining the form of a third global
quantity Q, which is argued to be approximately conserved during
collisionless collapse (in addition to the total energy and the total
number of stars).
The family of models is found to have the following properties.
The intrinsic density profile beyond the half-mass radius rM is
basically universal and independent of Ψ. The projected
density profiles are well fitted by the law, with n
ranging from 2.5 to 8.5, dependent on Ψ, with n close
to 4 for concentrated models. All models exhibit radial anisotropy
in the pressure tensor, especially in their outer parts, already
significant at
. At fixed values of ν, models
with lower Ψ are more anisotropic; at fixed values of Ψ,
models with lower ν are more concentrated and more
anisotropic. When the global amount of anisotropy, measured by
, is large, the models are unstable with respect to the
radial-orbit instability; still, a wide region of parameter space
(i.e., sufficiently high values of Ψ, for
) is
covered by models that are dynamically stable; for these, the line
profiles (line-of-sight velocity distribution) are Gaussian at the
level, with a general trend of positive values of h4 at
radii larger than the effective radius Re.
Key words: stellar dynamics / galaxies: evolution / galaxies: formation / galaxies: kinematics and dynamics / galaxies: structure
© ESO, 2005
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