1,009 research outputs found
Environmental effect on the subhalo abundance -- a solution to the missing dwarf problem
Recent high-resolution simulations of the formation of dark-matter halos have
shown that the distribution of subhalos is scale-free, in the sense that if
scaled by the velocity dispersion of the parent halo, the velocity distribution
function of galaxy-sized and cluster-sized halos are identical. For
cluster-sized halos, simulation results agreed well with observations.
Simulations, however, predicted far too many subhalos for galaxy-sized halos.
Our galaxy has several tens of known dwarf galaxies. On the other hands,
simulated dark-matter halos contain thousands of subhalos. We have performed
simulation of a single large volume and measured the abundance of subhalos in
all massive halos. We found that the variation of the subhalo abundance is very
large, and those with largest number of subhalos correspond to simulated halos
in previous studies. The subhalo abundance depends strongly on the local
density of the background. Halos in high-density regions contain large number
of subhalos. Our galaxy is in the low-density region. For our simulated halos
in low-density regions, the number of subhalos is within a factor of three to
that of our galaxy. We argue that the ``missing dwarf problem'' is not a real
problem but caused by the biased selection of the initial conditions in
previous studies, which were not appropriate for field galaxies.Comment: 8 pages, 5 figures, higher resolution run added, accepted by PAS
GreeM : Massively Parallel TreePM Code for Large Cosmological N-body Simulations
In this paper, we describe the implementation and performance of GreeM, a
massively parallel TreePM code for large-scale cosmological N-body simulations.
GreeM uses a recursive multi-section algorithm for domain decomposition. The
size of the domains are adjusted so that the total calculation time of the
force becomes the same for all processes. The loss of performance due to
non-optimal load balancing is around 4%, even for more than 10^3 CPU cores.
GreeM runs efficiently on PC clusters and massively-parallel computers such as
a Cray XT4. The measured calculation speed on Cray XT4 is 5 \times 10^4
particles per second per CPU core, for the case of an opening angle of
\theta=0.5, if the number of particles per CPU core is larger than 10^6.Comment: 13 pages, 11 figures, accepted by PAS
4.45 Pflops Astrophysical N-Body Simulation on K computer -- The Gravitational Trillion-Body Problem
As an entry for the 2012 Gordon-Bell performance prize, we report performance
results of astrophysical N-body simulations of one trillion particles performed
on the full system of K computer. This is the first gravitational trillion-body
simulation in the world. We describe the scientific motivation, the numerical
algorithm, the parallelization strategy, and the performance analysis. Unlike
many previous Gordon-Bell prize winners that used the tree algorithm for
astrophysical N-body simulations, we used the hybrid TreePM method, for similar
level of accuracy in which the short-range force is calculated by the tree
algorithm, and the long-range force is solved by the particle-mesh algorithm.
We developed a highly-tuned gravity kernel for short-range forces, and a novel
communication algorithm for long-range forces. The average performance on 24576
and 82944 nodes of K computer are 1.53 and 4.45 Pflops, which correspond to 49%
and 42% of the peak speed.Comment: 10 pages, 6 figures, Proceedings of Supercomputing 2012
(http://sc12.supercomputing.org/), Gordon Bell Prize Winner. Additional
information is http://www.ccs.tsukuba.ac.jp/CCS/eng/gbp201
The Abundance and Structure of Subhaloes near the Free Streaming Scale and Their Impact on Indirect Dark Matter Searches
The free streaming motion of dark matter particles imprints a cutoff in the
matter power spectrum and set the scale of the smallest dark matter halo.
Recent cosmological -body simulations have shown that the central density
cusp is much steeper in haloes near the free streaming scale than in more
massive haloes. Here, we study the abundance and structure of subhaloes near
the free streaming scale at very high redshift using a suite of unprecedentedly
large cosmological -body simulations, over a wide range of the host halo
mass. The subhalo abundance is suppressed strongly below the free streaming
scale, but the ratio between the subhalo mass function in the cutoff and no
cutoff simulations is well fitted by a single correction function regardless of
the host halo mass and the redshift. In subhaloes, the central slopes are
considerably shallower than in field haloes, however, are still steeper than
that of the NFW profile. Contrary, the concentrations are significantly larger
in subhaloes than haloes and depend on the subhalo mass. We compare two methods
to extrapolate the mass-concentration relation of haloes and subhaloes to z=0
and provide a new simple fitting function for subhaloes, based on a suite of
large cosmological -body simulations. Finally, we estimate the annihilation
boost factor of a Milky-Way sized halo to be between 1.8 and 6.2.Comment: 11 pages, 9 figures, accepted by MNRA
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