65 research outputs found
Adding Long Wavelength Modes to an -Body Simulation
We present a new method to add long wavelength power to an evolved -body
simulation, making use of the Zel'dovich (1970) approximation to change
positions and velocities of particles. We describe the theoretical framework of
our technique and apply it to a PM cosmological simulation performed on a
cube of Mpc on a side, obtaining a new ``simulation'' of Mpc on a
side. We study the effect of the power added by long waves by mean of several
statistics of the density and velocity field, and suggest possible applications
of our method to the study of the large-scale structure of the universe.Comment: Revised version, shortened. 15 pages without figures. Accepted for
publication in the Astrophysical Journal. Paper and 11 Figures available as
.ps.gz files by anonymous ftp at ftp://ftp.mpa-garching.mpg.de/pub/bepi/MA
Hydrodynamic Simulations of Galaxy Formation
This review is a short introduction to numerical hydrodynamics in a
cosmological context, intended for the non specialist. The main processes
relevant to galaxy formation are first presented. The fluid equations are then
introduced, and their implementation in numerical codes by Eulerian grid based
methods and by Smooth Particle Hydrodynamics is sketched. As an application, I
finally show some results from an SPH simulation of a galaxy cluster.Comment: uuencoded gzipped latex file, 8 pages with 2 figures. Invited talk to
appear in the Proceedings of the XXXIst Rencontres de Moriond "Dark Matter in
Cosmology, Quantum Measurements, Experimental Gravitation" held in Les Arcs,
January 1996. Editions Frontiere
Constraining the distribution of dark matter in dwarf spheroidal galaxies with stellar tidal streams
We use high-resolution N-body simulations to follow the formation and
evolution of tidal streams associated to dwarf spheroidal galaxies (dSphs). The
dSph models are embedded in dark matter (DM) haloes with either a
centrally-divergent 'cusp', or an homogeneous-density 'core'. In agreement with
previous studies, we find that as tides strip the galaxy the evolution of the
half-light radius and the averaged velocity dispersion follows well-defined
tracks that are mainly controlled by the amount of mass lost. Crucially, the
evolutionary tracks behave differently depending on the shape of the DM
profile: at a fixed remnant mass, dSphs embedded in cored haloes have larger
sizes and higher velocity dispersions than their cuspy counterparts. The
divergent evolution is particularly pronounced in galaxies whose stellar
component is strongly segregated within their DM halo and becomes more
disparate as the remnant mass decreases. Our analysis indicates that the DM
profile plays an important role in defining the internal dynamics of tidal
streams. We find that stellar streams associated to cored DM models have
velocity dispersions that lie systematically above their cuspy counterparts.
Our results suggest that the dynamics of streams with known dSph progenitors
may provide strong constraints on the distribution of DM on the smallest
galactic scales.Comment: 5 pages, 4 figure
Survival of Substructure within Dark Matter Haloes
Using high resolution cosmological N-body simulations, we investigate the
survival of dark matter satellites falling into larger haloes. Satellites
preserve their identity for some time after merging. We compute their loss of
mass, energy and angular momentum as dynamical friction, tidal forces and
collisions with other satellites dissolve them. We also analyse the evolution
of their internal structure. Satellites with less than a few per cent the mass
of the main halo may survive for several billion years, whereas larger
satellites rapidly sink into the center of the main halo potential well and
lose their identity. Penetrating encounters between satellites are frequent and
may lead to significant mass loss and disruption. Only a minor fraction of
cluster mass (10 per cent on average) is bound to substructure at most
redshifts of interest. We discuss the application of these results to the
survival and extent of dark matter haloes associated with cluster galaxies, and
to interactions between galaxies in clusters. We find that 35-40 per cent of
galaxy dark matter haloes are disrupted by the present time. The fraction of
satellites undergoing close encounters is similar to the fraction of
interacting or merging galaxies in clusters at moderate redshift.Comment: 16 pages, Latex, 14 Postscript figures. Submitted to MNRAS.
Postscript version also available at http://www.mpa-garching.mpg.de/~bep
The Rise and Fall of Satellites in Galaxy Clusters
We use N-body simulations to study the infall of dark matter haloes onto rich
clusters of galaxies. After identification of all cluster progenitors in the
simulations, we select those haloes which accrete directly onto the main
cluster progenitor. We construct the mass function of these merging satellites,
and calculate the main orbital parameters for the accreted lumps. The average
circularity of the orbits is epsilon = 0.5, while either radial or almost
circular orbits are equally avoided. More massive satellites move along
slightly more eccentric orbits, with lower specific angular momentum and a
smaller pericentre. We find that the infall of satellites onto the main cluster
progenitor has a very anisotropic distribution. This anisotropy is to a large
extent responsible for the shape and orientation of the final cluster and of
its velocity ellipsoid. At the end of the simulations, the major axis of the
cluster is aligned both with that of its velocity ellipsoid, and with the major
axis of the ellipsoid defined by the satellite infall pattern, to 30 degrees on
average. We also find that, in lower mass clusters, a higher fraction of the
final virial mass is provided by small, dense satellites. These sink to the
centre of the parent cluster and so enhance its central density. This mechanism
is found to be partially responsible for the correlation between halo masses
and characteristic overdensities, recently highlighted by Navarro, Frenk &
White (1996).Comment: Revised to match the published versio
Ellipsoidal halo finders and implications for models of triaxial halo formation
We describe an algorithm for identifying ellipsoidal haloes in numerical
simulations, and quantify how the resulting estimates of halo mass and shape
differ with respect to spherical halo finders. Haloes become more prolate when
fit with ellipsoids, the difference being most pronounced for the more
aspherical objects. Although the ellipsoidal mass is systematically larger,
this is less than 10% for most of the haloes. However, even this small
difference in mass corresponds to a significant difference in shape. We
quantify these effects also on the initial mass and deformation tensors, on
which most models of triaxial collapse are based. By studying the properties of
protohaloes in the initial conditions, we find that models in which protohaloes
are identified in Lagrangian space by three positive eigenvalues of the
deformation tensor are tenable only at the masses well-above . The
overdensity within almost any protohalo is larger than the critical
value associated with spherical collapse (increasing as mass decreases); this
is in good qualitative agreement with models which identify haloes requiring
that collapse have occured along all three principal axes, each axis having
turned around from the universal expansion at a different time. The
distributions of initial values are in agreement with the simplest predictions
associated with ellipsoidal collapse, assuming initially spherical protohaloes,
collapsed around random positions which were sufficiently overdense. However,
most protohaloes are not spherical and departures from sphericity increase as
protohalo mass decreases. [Abridged]Comment: 18 pages, 17 figures, accepted for publication in MNRA
The Structure and Dynamical Evolution of Dark Matter Halos
(Shortened) We use N-body simulations to investigate the structure and
dynamical evolution of dark matter halos in galaxy clusters. Our sample
consists of nine massive halos from an EdS universe with scale free power
spectrum and n = -1. Halos are resolved by ~20000 particles each, with a
dynamical resolution of 20-25 kpc. Large scale tidal fields are included up to
L=150 Mpc using background particles. The halo formation process can be
characterized by the alternation of two dynamical configurations: a merging
phase and a relaxation phase, defined by their signature on the evolution of
the total mass and rms velocity. Halos spend on average one 1/3 of their
evolution in the merging phase and 2/3 in the relaxation phase. Using this
definition, we study the density profiles and their change during the halo
history. The average density profiles are fitted by the NFW analytical model
with an rms residual of 17% between the virial radius Rv and 0.01 Rv. The
Hernquist (1990) profiles fits the same halos with an rms residual of 26%. The
trend with mass of the scale radius of these fits is marginally consistent with
that found by Cole & Lacey (1996): in comparison our halos are more centrally
concentrated, and the relation between scale radius and halo mass is slightly
steeper. We find a moderately large scatter in this relation, due both to
dynamical evolution within halos and to fluctuations in the halo population. We
analyze the dynamical equilibrium of our halos using the Jeans' equation, and
find that on average they are approximately in equilibrium within their virial
radius. Finally, we find that the projected mass profiles of our simulated
halos are in very good agreement with the profiles of three rich galaxy
clusters derived from strong and weak gravitational lensing observations.Comment: 20 pages, Latex, with all figures included. Modified to match the
published versio
The galaxy velocity field and CDM models
It is generally accepted that some kind of non-baryonic dark matter accounts for most of the mass density of the universe. Considering such a component has become, in the last decade, a key ingredient in current theories of structure formation. In particular, the Cold Dark Matter (CDM) scenario has proven to be quite successful in explaining most of the observed properties of galaxies and of their large-scale distribution. The standard CDM model is characterized by a primordial Zel'dovich spectrum, of random-phase adiabatic perturbations in a universe with density parameter omega sub 0 = 1 and vanishing cosmological constant. This poster paper presents an analysis of observational data on peculiar motion of optical galaxies in comparison to the predictions of CDM models where the assumptions of the standard scenario: omega sub 0 = 1, n = 1, and bias parameter b = 1 are relaxed. In particular, CDM models with 0 less than n less than 1 and 0.4 less than omega sub 0 less than 1 are considered
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