349 research outputs found
Reply to Melott's Comment on ``Discreteness Effects in Lambda Cold Dark Matter Simulations: A Wavelet-Statistical View'' by Romeo et al
Melott has made pioneering studies of the effects of particle discreteness in
N-body simulations, a fundamental point that needs careful thought and analysis
since all such simulations suffer from numerical noise arising from the use of
finite-mass particles. Melott (arXiv:0804.0589) claims that the conclusions of
our paper (arXiv:0804.0294) are essentially equivalent to those of his earlier
work. Melott is wrong: he has jumped onto one of our conclusions and
interpreted that in his own way. Here we point out the whys and the wherefores
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
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
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
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 ÎCDM 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 Ă 108 Mâ 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
Simultaneous ram pressure and tidal stripping; how dwarf spheroidals lost their gas
We perform high-resolution N-Body+SPH simulations of gas-rich dwarf galaxy
satellites orbiting within a Milky Way-sized halo and study for the first time
the combined effects of tides and ram pressure. The structure of the galaxy
models and the orbital configurations are chosen in accordance to those
expected in a LCDM Universe.While tidal stirring of disky dwarfs produces
objects whose stellar structure and kinematics resembles that of dwarf
spheroidals after a few orbits, ram pressure stripping is needed to entirely
remove their gas component. Gravitational tides can aid ram pressure stripping
by diminishing the overall potential of the dwarf, but tides also induce bar
formation which funnels gas inwards making subsequent stripping more difficult.
This inflow is particularly effective when the gas can cool radiatively.
Assuming a low density of the hot Galactic corona consistent with observational
constraints, dwarfs with V_{peak} < 30 km/s can be completely stripped of their
gas content on orbits with pericenters of 50 kpc or less. Instead, dwarfs with
more massive dark haloes and V_{peak} > 30 km/s lose most or all of their gas
content only if a heating source keeps the gas extended, partially
counteracting the bar-driven inflow. We show that the ionizing radiation from
the cosmic UV background at z > 2 can provide the required heating. In these
objects most of the gas is removed or becomes ionized at the first pericenter
passage,explaining the early truncation of the star formation observed in Draco
and Ursa Minor. The stripped gas breaks up into individual clouds pressure
confined by the outer gaseous medium that have masses, sizes and densities
comparable to the HI clouds recently discovered around M31.(abridged)Comment: 21 pages, 17 figures, submitted to MNRAS. High resolution version of
the paper and movies can be found at
http://www-theorie.physik.unizh.ch/~chiar
The fate of planetesimal discs in young open clusters: implications for 1I/âOumuamua, the Kuiper belt, the Oort cloud, and more
We perform N-body simulations of the early phases of open cluster evolution including a large population of planetesimals, initially arranged in Kuiper-belt like discs around each star. Using a new, fourth-order, and time-reversible N-body code on Graphics Processing Units (GPUs), we evolve the whole system under the stellar gravity, i.e. treating planetesimals as test particles, and consider two types of initial cluster models, similar to IC348 and the Hyades, respectively. In both cases, planetesimals can be dynamically excited, transferred between stars, or liberated to become free-floating (such as A/2017 U1 or âOumuamua) during the early cluster evolution. We find that planetesimals captured from another star are not necessarily dynamically distinct from those native to a star. After an encounter, both native and captured planetesimals can exhibit aligned periastrons, qualitatively similar to that seen in the Solar system and commonly thought to be the signature of Planet 9. We discuss the implications of our results for both our Solar system and exoplanetary systems
The gravitational and hydrodynamical interaction between the LMC and the Galaxy
We use high resolution N-Body/SPH simulations to study the hydrodynamical and
gravitational interaction between the Large Magellanic Cloud and the Milky Way.
We model the dark and hot extended halo components as well as the
stellar/gaseous disks of the two galaxies. Both galaxies are embedded in
extended cuspy LCDM dark matter halos. We follow the previous four Gyrs 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 disk, 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. Ram-pressure
from a low density ionised halo is then sufficient to remove 1.4e8 Msolar of
gas from the LMC's disk forming a great circle trailing stream around the
Galaxy. The column density of stripped gas falls by two orders of magnitude 100
degrees from LMC. The LMC does not induce any response in the Milky Way disk.
On the contrary, the tides raised by the Milky Way determine the truncation of
the satellite at about 11 kpc. After several Gyrs of interaction the gas disk
of the LMC is smaller than the stellar disk due to ram pressure and its size
compares well with the observational data.Comment: 12 pages, 15 figures. Submitted to MNRAS. Movies and high resolution
images are available at http://www-theorie.physik.unizh.ch/~chiara/lmc.
Corrected typo
- âŠ