6,280 research outputs found
A numerical investigation of the Milky Way and of the satellite tidal debris in the Galactic environment
I address the satellite debris distribution in the Milky Way (MW) environment by means of N-body simulations, combining full N-body MW models with realistic high-resolution N-body satellites and cosmologically motivated initial conditions. For the choice of the code, I perform a benchmark on previous N-body simulations of the MW environment, proving that Gadget-2 performs similar to more modern codes, and that Gadget-4 offers an improved momentum conservation compared to Gadget-2. Then, with Gadget-4 I simulate the satellite debris distribution in the MW environment. Stars are stripped less efficiently than dark matter (DM) from the satellites and larger fractions of stellar debris are found in the MW central regions, where the stellar and DM debris have different orientations and do not change them significantly if the MW disc is initially tilted. I conclude that the satellite initial conditions have more impact than the disc on the local debris orientation, and that the DM and stellar debris are spatially uncorrelated. Finally, I present a study of the bar in an N-body MW model that matches the observational constraints of the Galaxy. The strong bar formed in this simulation is a slow rotator that influences the local disc kinematics and dynamics and does not present significant buckling
Neutrinos in Non-linear Structure Formation - a Simple SPH Approach
We present a novel method for implementing massive neutrinos in N-body
simulations. Instead of sampling the neutrino velocity distribution by
individual point particles we take neutrino free-streaming into account by
treating it as an effective redshift dependent sound speed in a perfect
isothermal fluid, and assume a relation between the sound speed and velocity
dispersion of the neutrinos. Although the method fails to accurately model the
true neutrino power spectrum, it is able to calculate the total matter power
spectrum to the same accuracy as more complex hybrid neutrino methods, except
on very small scales. We also present an easy way to update the publicly
available Gadget-2 version with this neutrino approximation.Comment: 13 pages, 7 figure
The Shape-Alignment relation in CDM Cosmic Structures
In this paper we study the supercluster - cluster morphological properties
using one of the largest ( SPH+N-body simulations of large
scale structure formation in a CDM model, based on the publicly
available code GADGET. We find that filamentary (prolate-like) shapes are the
dominant supercluster and cluster dark matter halo morphological feature, in
agreement with previous studies. However, the baryonic gas component of the
clusters is predominantly spherical. We investigate the alignment between
cluster halos (using either their DM or baryonic components) and their parent
supercluster major-axis orientation, finding that clusters show such a
preferential alignment. Combining the shape and the alignment statistics, we
also find that the amplitude of supercluster - cluster alignment increases
although weakly with supercluster filamentariness.Comment: Accepted for puplication in MNRAS, 10 pages, 15 figure
The cosmological simulation code GADGET-2
We discuss the cosmological simulation code GADGET-2, a new massively
parallel TreeSPH code, capable of following a collisionless fluid with the
N-body method, and an ideal gas by means of smoothed particle hydrodynamics
(SPH). Our implementation of SPH manifestly conserves energy and entropy in
regions free of dissipation, while allowing for fully adaptive smoothing
lengths. Gravitational forces are computed with a hierarchical multipole
expansion, which can optionally be applied in the form of a TreePM algorithm,
where only short-range forces are computed with the `tree'-method while
long-range forces are determined with Fourier techniques. Time integration is
based on a quasi-symplectic scheme where long-range and short-range forces can
be integrated with different timesteps. Individual and adaptive short-range
timesteps may also be employed. The domain decomposition used in the
parallelisation algorithm is based on a space-filling curve, resulting in high
flexibility and tree force errors that do not depend on the way the domains are
cut. The code is efficient in terms of memory consumption and required
communication bandwidth. It has been used to compute the first cosmological
N-body simulation with more than 10^10 dark matter particles, reaching a
homogeneous spatial dynamic range of 10^5 per dimension in a 3D box. It has
also been used to carry out very large cosmological SPH simulations that
account for radiative cooling and star formation, reaching total particle
numbers of more than 250 million. We present the algorithms used by the code
and discuss their accuracy and performance using a number of test problems.
GADGET-2 is publicly released to the research community.Comment: submitted to MNRAS, 31 pages, 20 figures (reduced resolution), code
available at http://www.mpa-garching.mpg.de/gadge
Is the Sgr dSph a dark matter dominated system?
We study the evolution of possible progenitors of Sgr dSph}using several
numerical N-body simulations of different dwarf spheroidal galaxies both with
and without dark matter, as they orbit the Milky Way. The barionic and dark
components of the dwarfs were made obeying a Plummer and NFW potentials of one
million particles respectively. The Milky Way was modeled like a tree-component
rigid potential and the simulations were performed using a modified Gadget-2
code. We found that none of the simulated galaxies without dark matter
reproduced the physical properties observed in Sgr dSph, suggesting that, at
the beginning of its evolution, Sgr dSph might have been immersed in a dark
matter halo.
The simulations of progenitors immersed in dark matter halos suggest that Sgr
dSph at its beginning might have been an extended system, i.e. its Plummer
radius could have had a value approximated to 1.2 kpc or higher; furthermore,
this galaxy could have been immersed in a dark halo with a mass higher than
10^8 solar masses. These results are important for the construction of a model
of the formation of Sgr dSph.Comment: 13 pages, 6 figures, New Astronomy - accepte
Dynamics of barred galaxies: effects of disk height
We study dynamics of bars in models of disk galaxies embeded in realistic
dark matter halos. We find that disk thickness plays an important, if not
dominant, role in the evolution and structure of the bars. We also make
extensive numerical tests of different N-body codes used to study bar dynamics.
Models with thick disks typically used in this type of modeling
(height-to-length ratio hz/Rd=0.2) produce slowly rotating, and very long,
bars. In contrast, more realistic thin disks with the same parameters as in our
Galaxy (hz/Rd= 0.1) produce bars with normal length Rbar approx R_d, which
rotate quickly with the ratio of the corotation radius to the bar radius
1.2-1.4 compatible with observations. Bars in these models do not show a
tendency to slow down, and may lose as little as 2-3 percent of their angular
momentum due to dynamical friction with the dark matter over cosmological time.
We attribute the differences between the models to a combined effect of high
phase-space density and smaller Jeans mass in the thin disk models, which
result in the formation of a dense central bulge. Special attention is paid to
numerical effects such as the accuracy of orbital integration, force and mass
resolution. Using three N-body codes -- Gadget, ART, and Pkdgrav -- we find
that numerical effects are very important and, if not carefully treated, may
produce incorrect and misleading results. Once the simulations are performed
with sufficiently small time-steps and with adequate force and mass resolution,
all the codes produce nearly the same results: we do not find any systematic
deviations between the results obtained with TREE codes (Gadget and Pkdgrav)
and with the Adaptive-Mesh-Refinement (ART) code.Comment: 15 pages, 14 plots, submitted to MNRA
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