1,301 research outputs found
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
Tomography of Collisionless Stellar Systems
In this paper the concept of tomography of a collisionless stellar system of
general shape is introduced, and a generalization of the Projected Virial
Theorem is obtained. Applying the tomographic procedure we then derive a new
family of virial equations which coincides with the already known ones for
spherically symmetric systems. This result is obtained without any use of
explicit expressions for the line-of-sight velocity dispersion, or spherical
coordinate system.Comment: BAP-06-1994-016-OAB. 7 pages, postscript file. In press on Celestial
Mechanic
Galactic cannibalism in the galaxy cluster C0337-2522 at z=0.59
According to the galactic cannibalism model, cD galaxies are formed in the
center of galaxy clusters by merging of massive galaxies and accretion of
smaller stellar systems: however, observational examples of the initial phases
of this process are lacking. We have identified a strong candidate for this
early stage of cD galaxy formation: a group of five elliptical galaxies in the
core of the X-ray cluster C0337-2522 at redshift z=0.59. With the aid of
numerical simulations, in which the galaxies are represented by N-body systems,
we study their dynamical evolution up to z=0; the cluster dark matter
distribution is also described as a N-body system. We find that a multiple
merging event in the considered group of galaxies will take place before z=0
and that the merger remnant preserves the Fundamental Plane and the
Faber-Jackson relations, while its behavior with respect to the Mbh-sigma
relation is quite sensitive to the details of black hole merging [abridged].Comment: 30 pages, 7 figures, MNRAS (accepted
Decoupled and inhomogeneous gas flows in S0 galaxies
A recent analysis of the "Einstein" sample of early-type galaxies has
revealed that at any fixed optical luminosity Lb S0 galaxies have lower mean
X-ray luminosity Lx per unit Lb than ellipticals. Following a previous
analytical investigation of this problem (Ciotti & Pellegrini 1996), we have
performed 2D numerical simulations of the gas flows inside S0 galaxies in order
to ascertain the effectiveness of rotation and/or galaxy flattening in reducing
the Lx/Lb ratio. The flow in models without SNIa heating is considerably
ordered, and essentially all the gas lost by the stars is cooled and
accumulated in the galaxy center. If rotation is present, the cold material
settles in a disk on the galactic equatorial plane. Models with a time
decreasing SNIa heating host gas flows that can be much more complex. After an
initial wind phase, gas flows in energetically strongly bound galaxies tend to
reverse to inflows. This occurs in the polar regions, while the disk is still
in the outflow phase. In this phase of strong decoupling, cold filaments are
created at the interface between inflowing and outflowing gas. Models with more
realistic values of the dynamical quantities are preferentially found in the
wind phase with respect to their spherical counterparts of equal Lb. The
resulting Lx of this class of models is lower than in spherical models with the
same Lb and SNIa heating. At variance with cooling flow models, rotation is
shown to have only a marginal effect in this reduction, while the flattening is
one of the driving parameters for such underluminosity, in accordance with the
analytical investigation.Comment: 32 pages LaTex file, plus 5 .ps figures and macro aasms4.sty --
Accepted on Ap
Modelling elliptical galaxies: phase-space constraints on two-component (gamma1,gamma2) models
In the context of the study of the properties of the mutual mass distribution
of the bright and dark matter in elliptical galaxies, present a family of
two-component, spherical, self-consistent galaxy models, where one density
distribution follows a gamma_1 profile, and the other a gamma_2 profile
[(gamma_1,gamma_2) models], with different total masses and ``core'' radii. A
variable amount of Osipkov-Merritt (radial) orbital anisotropy is allowed in
both components. For these models, I derive analytically the necessary and
sufficient conditions that the model parameters must satisfy in order to
correspond to a physical system. Moreover, the possibility of adding a black
hole at the center of radially anisotropic gamma models is discussed,
determining analytically a lower limit of the anisotropy radius as a function
of gamma. The analytical phase-space distribution function for (1,0) models is
presented, together with the solution of the Jeans equations and the quantities
entering the scalar virial theorem. It is proved that a globally isotropic
gamma=1 component is consistent for any mass and core radius of the
superimposed gamma=0 model; on the contrary, only a maximum value of the core
radius is allowed for the gamma=0 model when a gamma=1 density distribution is
added. The combined effects of mass concentration and orbital anisotropy are
investigated, and an interesting behavior of the distribution function of the
anisotropic gamma=0 component is found: there exists a region in the parameter
space where a sufficient amount of anisotropy results in a consistent model,
while the structurally identical but isotropic model would be inconsistent.Comment: 29 pages, LaTex, plus 5 .eps figures and macro aaspp4.sty - accepted
by ApJ, main journa
Asymmetric Gravitational Lenses in TeVeS and Application to the Bullet Cluster
Aims: We explore the lensing properties of asymmetric matter density
distributions in Bekenstein's Tensor-Vector-Scalar theory (TeVeS). Methods:
Using an iterative Fourier-based solver for the resulting non-linear scalar
field equation, we numerically calculate the total gravitational potential and
derive the corresponding TeVeS lensing maps. Results: Considering variations on
rather small scales, we show that the lensing properties significantly depend
on the lens's extent along the line of sight. Furthermore, all simulated TeVeS
convergence maps strongly track the dominant baryonic components, non-linear
effects, being capable of counteracting this trend, turn out to be very small.
Setting up a toy model for the cluster merger 1E0657-558, we infer that TeVeS
cannot explain observations without assuming an additional dark mass component
in both cluster centers, which is in accordance with previous work.Comment: LaTex, 14 pages, 10 figures, references added, 2 figures removed,
minor text changes to fit accepted version (A&A
Radiative feedback from massive black holes in elliptical galaxies. AGN flaring and central starburst fueled by recycled gas
The importance of the radiative feedback from massive black holes at the
centers of elliptical galaxies is not in doubt, given the well established
relations among electromagnetic output, black hole mass and galaxy optical
luminosity. We show how this AGN radiative output affects the hot ISM of an
isolated elliptical galaxy with the aid of a high-resolution hydrodynamical
code, where the cooling and heating functions include photoionization plus
Compton heating. We find that radiative heating is a key factor in the
self-regulated coevolution of massive black holes and their host galaxies and
that 1) the mass accumulated by the central black hole is limited by feedback
to the range observed today, and 2) relaxation instabilities occur so that duty
cycles are small enough (~0.03) to account for the very small fraction of
massive ellipticals observed to be in the "on" -QSO- phase, when the accretion
luminosity approaches the Eddington luminosity. The duty cycle of the hot
bubbles inflated at the galaxy center during major accretion episodes is of the
order of 0.1-0.4. Major accretion episodes caused by cooling flows in the
recycled gas produced by normal stellar evolution trigger nuclear starbursts
coincident with AGN flaring. During such episodes the central sources are often
obscured; but overall, in the bursting phase (1<z<3), the duty cycle of the
black hole in its "on" phase is of the order of percents and it is unobscured
approximately one-third of the time. Mechanical energy output from
non-relativistic gas winds integrates to 2.3 10^{59} erg, with most of it
caused by broadline AGN outflows. [abridged]Comment: ApJ resubmitted. 48 pages, 14 figures (some of them new, bitmapped,
low resolution). New references added, typos correcte
The Mass Assembly History of Spheroidal Galaxies: Did Newly-Formed Systems Arise Via Major Mergers?
We examine the properties of a morphologically-selected sample of 0.4<z<1.0
spheroidal galaxies in the GOODS fields in order to ascertain whether their
increase in abundance with time arises primarily from mergers. To address this
question we determine scaling relations between the dynamical mass determined
from stellar velocity dispersions, and the stellar mass determined from optical
and infrared photometry. We exploit these relations across the larger sample
for which we have stellar masses in order to construct the first statistically
robust estimate of the evolving dynamical mass function over 0<z<1. The trends
observed match those seen in the stellar mass functions of Bundy et al. 2005
regarding the top-down growth in the abundance of spheroidal galaxies. By
referencing our dynamical masses to the halo virial mass we compare the growth
rate in the abundance of spheroidals to that predicted by the assembly of dark
matter halos. Our comparisons demonstrate that major mergers do not fully
account for the appearance of new spheroidals since z~1 and that additional
mechanisms, such as morphological transformations, are required to drive the
observed evolution.Comment: Accepted to ApJL; New version corrects the Millennium merger
predictions--further details at
http://www.astro.utoronto.ca/~bundy/millennium
Weak homology of elliptical galaxies
We start by studying a small set of objects characterized by photometric
profiles that have been pointed out to deviate significantly from the standard
R^{1/4} law. For these objects we confirm that a generic R^{1/n} law, with n a
free parameter, can provide superior fits (the best-fit value of n can be lower
than 2.5 or higher than 10), better than those that can be obtained by a pure
R^{1/4} law, by an R^{1/4}+exponential model, and by other dynamically
justified self--consistent models. Therefore, strictly speaking, elliptical
galaxies should not be considered homologous dynamical systems. Still, a case
for "weak homology", useful for the interpretation of the Fundamental Plane of
elliptical galaxies, could be made if the best-fit parameter n, as often
reported, correlates with galaxy luminosity L, provided the underlying
dynamical structure also follows a systematic trend with luminosity. We
demonstrate that this statement may be true even in the presence of significant
scatter in the correlation n(L). Preliminary indications provided by a set of
"data points" associated with a sample of 14 galaxies suggest that neither the
strict homology nor the constant stellar mass--to--light solution are a
satisfactory explanation of the observed Fundamental Plane (abridged).Comment: 34 pages, 11 figures, accepted by Astronomy and Astrophysic
Hydrostatic models for the rotation of extra-planar gas in disk galaxies
We show that fluid stationary models are able to reproduce the observed,
negative vertical gradient of the rotation velocity of the extra-planar gas in
spiral galaxies. We have constructed models based on the simple condition that
the pressure of the medium does not depend on density alone (baroclinic instead
of barotropic solutions: isodensity and isothermal surfaces do not coincide).
As an illustration, we have successfully applied our method to reproduce the
observed velocity gradient of the lagging gaseous halo of NGC 891. The fluid
stationary models discussed here can describe a hot homogeneous medium as well
as a "gas" made of discrete, cold HI clouds with an isotropic velocity
dispersion distribution. Although the method presented here generates a density
and velocity field consistent with observational constraints, the stability of
these configurations remains an open question.Comment: 12 pages, 9 figures. Accepted for publication in Astronomy and
Astrophysic
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