6,793 research outputs found
A class of self-gravitating accretion disks
We consider a class of steady-state self-gravitating accretion disks for
which efficient cooling mechanisms are assumed to operate so that the disk is
self-regulated at a condition of approximate marginal Jeans stability. In an
earlier paper, this scenario had been shown to lead naturally, in the absence
of a central point mass, to a self-similar solution characterized by a flat
rotation curve. In this article we investigate the entire parameter space
available for such self-regulated accretion disks and provide two non-trivial
extensions of the model. The first extension is that of a bimodal disk,
obtained by partially relaxing the self-regulation constraint, so that full
matching with an inner "standard" Keplerian accretion disk takes place. The
second extension is the construction of self-regulated accretion disks embedded
in a diffuse spherical "halo". The analysis is further strengthened by a
careful discussion of the vertical structure of the disk, in such a way that
the transition from self-gravity dominated to non-gravitating disks is covered
uniformly.Comment: To appear in A&
Dynamical properties of a family of collisionless models of elliptical galaxies
N-body simulations of collisionless collapse have offered important clues to
the construction of realistic stellar dynamical models of elliptical galaxies.
Such simulations confirm and quantify the qualitative expectation that rapid
collapse of a self-gravitating collisionless system, initially cool and
significantly far from equilibrium, leads to incomplete relaxation, that is to
a quasi-equilibrium configuration characterized by isotropic, quasi-Maxwellian
distribution of stellar orbits in the inner regions and by radially biased
anisotropic pressure in the outer parts. In earlier studies, as illustrated in
a number of papers several years ago (see Bertin et al. 1993 and references
therein), the attention was largely focused on the successful comparison
between the models (constructed under the qualitative clues offered by the
N-body simulations mentioned above) and the observations. In this paper we
revisit the problem of incomplete violent relaxation, by making a direct
comparison between the detailed properties of a family of distribution
functions and those of the products of collisionless collapse found in N-body
simulations.Comment: to appear in "Plasmas in the Laboratory and in the Universe: new
insights and new challenges", G. Bertin, D. Farina, R. Pozzoli eds., AIP
Conference Proceedings, Vol. XXX, pp. YY
Dynamical models and numerical simulations of incomplete violent relaxation
N-body simulations of collisionless collapse have offered important clues to
the construction of realistic stellar dynamical models of elliptical galaxies.
Such simulations confirm and quantify the qualitative expectation that rapid
collapse of a self-gravitating collisionless system, initially cool and
significantly far from equilibrium, leads to incomplete relaxation. In this
paper we revisit the problem, by comparing the detailed properties of a family
of distribution functions derived from statistical mechanics arguments to those
of the products of collisionless collapse found in N-body simulations.Comment: 4 pages, 2 figures, to appear in the proceedings of the 39th
Rencontres de Moriond, "Exploring the Universe," La Thuile, Italy, March 28 -
April 4, 200
The relative concentration of visible and dark matter in clusters of galaxies
[Abridged] We consider two clusters (A496 and Coma) that are representative
of the two classes of cool-core and non-cool-core clusters. We first refer to a
two-component dynamical model that ignores the contribution from the galaxy
density distribution and study the condition of hydrostatic equilibrium for the
hot intracluster medium (ICM) under the assumption of spherical symmetry, in
the presence of dark matter. We model the ICM density distribution in terms of
a standard -model with , i.e. with a distribution similar to
that of a regular isothermal sphere (RIS), and fit the observed X-ray
brightness profiles. With the explicit purpose of ignoring cosmological
arguments, we na\"ively assume that dark matter, if present, has an analogous
density distribution, with the freedom of two different density and length
scales. The relative distribution of visible and dark matter is then derived by
fitting the temperature data for the ICM under conditions of hydrostatic
equilibrium. For both clusters, we find that dark matter is more concentrated
with respect to visible matter. We then test whether the conclusion changes
significantly when dark matter is taken to be distributed according to
cosmologically favored density profiles and when the contribution of the mass
contained in galaxies is taken into account. Although the qualitative
conclusions remain unchanged, we find that the contribution of galaxies to the
mass budget is more important than generally assumed. We also show that,
without resorting to additional information on the small scale, it is not
possible to tell whether a density cusp is present or absent in these systems.
[Abridged]Comment: 13 pages, 3 figures, accepted for publication in Il Nuovo Cimento
Spiral density waves in the outer galactic gaseous discs
Deep HI observations of the outer parts of disc galaxies demonstrate the
frequent presence of extended, well-developed spiral arms far beyond the
optical radius. To understand the nature and the origin of such outer spiral
structure, we investigate the propagation in the outer gaseous disc of
large-scale spiral waves excited in the bright optical disc. Using
hydrodynamical simulations, we show that non-axisymmetric density waves,
penetrating in the gas through the outer Lindblad resonance, can exhibit
relatively regular spiral structures outside the bright optical stellar disc.
For low-amplitude structures, the results of numerical simulations match the
predictions of a simple WKB linear theory. The amplitude of spiral structure
increases rapidly with radius. Beyond optical radii, spirals become
nonlinear (the linear theory becomes quantitatively and qualitatively
inadequate) and unstable to Kelvin-Helmholtz instability. In numerical
simulations, in models for which gas is available very far out, spiral arms can
extend out to 25 disc scale-lengths. A comparison between the properties of the
models we have investigated and the observed properties of individual galaxies
may shed light into the problem of the amount and distribution of dark matter
in the outer halo.Comment: 12 pages, 13 figures (accepted for publication in MNRAS
Properties of quasi-relaxed stellar systems in an external tidal field
In a previous paper, we have constructed a family of self-consistent triaxial
models of quasi-relaxed stellar systems, shaped by the tidal field of the
hosting galaxy, as an extension of the well-known spherical King models. For a
given tidal field, the models are characterized by two physical scales (such as
total mass and central velocity dispersion) and two dimensionless parameters
(the concentration parameter and the tidal strength). The most significant
departure from spherical symmetry occurs when the truncation radius of the
corresponding spherical King model is of the order of the tidal radius, which,
for a given tidal strength, is set by the maximum concentration value admitted.
For such maximally extended (or "critical") models the outer boundary has a
generally triaxial shape, given by the zero-velocity surface of the relevant
Jacobi integral, which is basically independent of the concentration parameter.
In turn, the external tidal field can give rise to significant global
departures from spherical symmetry (as measured, for example, by the quadrupole
of the mass distribution of the stellar system) only for low-concentration
models, for which the allowed maximal value of the tidal strength can be
relatively high. In this paper we describe in detail the intrinsic and the
projected structure and kinematics of the models, covering the entire parameter
space, from the case of sub-critical (characterized by "underfilling" of the
relevant Roche volume) to that of critical models. The intrinsic properties can
be a useful starting point for numerical simulations and other investigations
that require initialization of a stellar system in dynamical equilibrium. The
projected properties are a key step in the direction of a comparison with
observed globular clusters and other candidate stellar systems.Comment: 13 pages, 11 figures, uses emulateapj.cls with apjfonts.sty. Accepted
for publication in The Astrophysical Journa
Relaxation of spherical systems with long-range interactions: a numerical investigation
The process of relaxation of a system of particles interacting with
long-range forces is relevant to many areas of Physics. For obvious reasons, in
Stellar Dynamics much attention has been paid to the case of 1/r^2 force law.
However, recently the interest in alternative gravities emerged, and
significant differences with respect to Newtonian gravity have been found in
relaxation phenomena. Here we begin to explore this matter further, by using a
numerical model of spherical shells interacting with an 1/r^alpha force law
obeying the superposition principle. We find that the virialization and
phase-mixing times depend on the exponent alpha, with small values of alpha
corresponding to longer relaxation times, similarly to what happens when
comparing for N-body simulations in classical gravity and in Modified Newtonian
Dynamics.Comment: 6 pages, 3 figures, accepted in the International Journal of
Bifurcation and Chao
A dynamical study of Galactic globular clusters under different relaxation conditions
We perform a systematic combined photometric and kinematic analysis of a
sample of globular clusters under different relaxation conditions, based on
their core relaxation time (as listed in available catalogs), by means of two
well-known families of spherical stellar dynamical models. Systems
characterized by shorter relaxation time scales are expected to be better
described by isotropic King models, while less relaxed systems might be
interpreted by means of non-truncated, radially-biased anisotropic f^(\nu)
models, originally designed to represent stellar systems produced by a violent
relaxation formation process and applied here for the first time to the study
of globular clusters. The comparison between dynamical models and observations
is performed by fitting simultaneously surface brightness and velocity
dispersion profiles. For each globular cluster, the best-fit model in each
family is identified, along with a full error analysis on the relevant
parameters. Detailed structural properties and mass-to-light ratios are also
explicitly derived. We find that King models usually offer a good
representation of the observed photometric profiles, but often lead to less
satisfactory fits to the kinematic profiles, independently of the relaxation
condition of the systems. For some less relaxed clusters, f^(\nu) models
provide a good description of both observed profiles. Some derived structural
characteristics, such as the total mass or the half-mass radius, turn out to be
significantly model-dependent. The analysis confirms that, to answer some
important dynamical questions that bear on the formation and evolution of
globular clusters, it would be highly desirable to acquire larger numbers of
accurate kinematic data-points, well distributed over the cluster field.Comment: 18 pages, 7 figures. Accepted for publication in Astronomy &
Astrophysic
On-off intermittency over an extended range of control parameter
We propose a simple phenomenological model exhibiting on-off intermittency
over an extended range of control parameter. We find that the distribution of
the 'off' periods has as a power-law tail with an exponent varying continuously
between -1 and -2, at odds with standard on-off intermittency which occurs at a
specific value of the control parameter, and leads to the exponent -3/2. This
non-trivial behavior results from the competition between a strong slowing down
of the dynamics at small values of the observable, and a systematic drift
toward large values.Comment: 4 pages, 3 figure
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