608 research outputs found
A universal velocity distribution of relaxed collisionless structures
Several general trends have been identified for equilibrated,
self-gravitating collisionless systems, such as density or anisotropy profiles.
These are integrated quantities which naturally depend on the underlying
velocity distribution function (VDF) of the system. We study this VDF through a
set of numerical simulations, which allow us to extract both the radial and the
tangential VDF. We find that the shape of the VDF is universal, in the sense
that it depends only on two things namely the dispersion (radial or tangential)
and the local slope of the density. Both the radial and the tangential VDF's
are universal for a collection of simulations, including controlled collisions
with very different initial conditions, radial infall simulation, and
structures formed in cosmological simulations.Comment: 13 pages, 6 figures; oversimplified analysis corrected; changed
abstract and conclusions; significantly extended discussio
Toward an accurate mass function for precision cosmology
Cosmological surveys aim to use the evolution of the abundance of galaxy
clusters to accurately constrain the cosmological model. In the context of
LCDM, we show that it is possible to achieve the required percent level
accuracy in the halo mass function with gravity-only cosmological simulations,
and we provide simulation start and run parameter guidelines for doing so. Some
previous works have had sufficient statistical precision, but lacked robust
verification of absolute accuracy. Convergence tests of the mass function with,
for example, simulation start redshift can exhibit false convergence of the
mass function due to counteracting errors, potentially misleading one to infer
overly optimistic estimations of simulation accuracy. Percent level accuracy is
possible if initial condition particle mapping uses second order Lagrangian
Perturbation Theory, and if the start epoch is between 10 and 50 expansion
factors before the epoch of halo formation of interest. The mass function for
halos with fewer than ~1000 particles is highly sensitive to simulation
parameters and start redshift, implying a practical minimum mass resolution
limit due to mass discreteness. The narrow range in converged start redshift
suggests that it is not presently possible for a single simulation to capture
accurately the cluster mass function while also starting early enough to model
accurately the numbers of reionisation era galaxies, whose baryon feedback
processes may affect later cluster properties. Ultimately, to fully exploit
current and future cosmological surveys will require accurate modeling of
baryon physics and observable properties, a formidable challenge for which
accurate gravity-only simulations are just an initial step.Comment: revised in response to referee suggestions, MNRAS accepte
The gravitational and hydrodynamical interaction between the Large Magellanic Cloud and the Galaxy
We use high-resolution N-body/smoothed particle hydrodynamic simulations to study the hydrodynamical and gravitational interaction between the Large Magellanic Cloud (LMC) and the Milky Way Galaxy. We model the dark and hot extended halo components as well as the stellar/gaseous discs of the two galaxies. Both galaxies are embedded in extended cuspy ΛCDM dark matter haloes. We follow the previous 4 Gyr of the LMC's orbit such that it ends up with the correct location and orientation on the sky. Tidal forces elongate the LMC's disc, forcing a bar and creating a strong warp and diffuse stellar halo, although very few stars become unbound. The stellar halo may account for some of the microlensing events observed towards the LMC. Ram pressure from a low-density ionized halo is then sufficient to remove 1.4 × 108 M⊙ of gas from the LMC's disc, forming a great circle trailing stream around the Galaxy. The column density of stripped gas falls by two orders of magnitude 100 degrees from the LMC and the radial velocity along the trailing stream agrees well with the observations. The LMC does not induce any response in the Milky Way disc. On the contrary, the tides raised by the Milky Way determine the truncation of the satellite at about 11 kpc. After several gigayears of interaction, the gas disc of the LMC is smaller than the stellar disc due to ram pressure, and its size and morphology compare well with the observational dat
The velocity anisotropy - density slope relation
One can solve the Jeans equation analytically for equilibrated dark matter
structures, once given two pieces of input from numerical simulations. These
inputs are 1) a connection between phase-space density and radius, and 2) a
connection between velocity anisotropy and density slope, the \alpha-\beta
relation. The first (phase-space density v.s. radius) has already been analysed
through several different simulations, however the second (\alpha-\beta
relation) has not been quantified yet. We perform a large set of numerical
experiments in order to quantify the slope and zero-point of the \alpha-\beta
relation. We find strong indication that the relation is indeed an attractor.
When combined with the assumption of phase-space being a power-law in radius,
this allows us to conclude that equilibrated dark matter structures indeed have
zero central velocity anisotropy \beta_0 = 0, central density slope of \alpha_0
= -0.8, and outer anisotropy of \beta_\infty = 0.5.Comment: 15 pages, 7 figure
Resolving the Structure of Cold Dark Matter Halos
We examine the effects of mass resolution and force softening on the density
profiles of cold dark matter halos that form within cosmological N-body
simulations. As we increase the mass and force resolution, we resolve
progenitor halos that collapse at higher redshifts and have very high
densities. At our highest resolution we have nearly 3 million particles within
the virial radius, several orders of magnitude more than previously used and we
can resolve more than one thousand surviving dark matter halos within this
single virialised system. The halo profiles become steeper in the central
regions and we may not have achieved convergence to a unique slope within the
inner 10% of the virialised region. Results from two very high resolution halo
simulations yield steep inner density profiles, . The
abundance and properties of arcs formed within this potential will be different
from calculations based on lower resolution simulations. The kinematics of
disks within such a steep potential may prove problematic for the CDM model
when compared with the observed properties of halos on galactic scales.Comment: Final version, to be published in the ApJLetter
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