1,702 research outputs found
The MultiDark Database: Release of the Bolshoi and MultiDark Cosmological Simulations
We present the online MultiDark Database -- a Virtual Observatory-oriented,
relational database for hosting various cosmological simulations. The data is
accessible via an SQL (Structured Query Language) query interface, which also
allows users to directly pose scientific questions, as shown in a number of
examples in this paper. Further examples for the usage of the database are
given in its extensive online documentation (www.multidark.org). The database
is based on the same technology as the Millennium Database, a fact that will
greatly facilitate the usage of both suites of cosmological simulations. The
first release of the MultiDark Database hosts two 8.6 billion particle
cosmological N-body simulations: the Bolshoi (250/h Mpc simulation box, 1/h kpc
resolution) and MultiDark Run1 simulation (MDR1, or BigBolshoi, 1000/h Mpc
simulation box, 7/h kpc resolution). The extraction methods for halos/subhalos
from the raw simulation data, and how this data is structured in the database
are explained in this paper. With the first data release, users get full access
to halo/subhalo catalogs, various profiles of the halos at redshifts z=0-15,
and raw dark matter data for one time-step of the Bolshoi and four time-steps
of the MultiDark simulation. Later releases will also include galaxy mock
catalogs and additional merging trees for both simulations as well as new large
volume simulations with high resolution. This project is further proof of the
viability to store and present complex data using relational database
technology. We encourage other simulators to publish their results in a similar
manner.Comment: 28 pages, 9 figures, submitted to New Astronom
Finite temperature crossover from a crystalline to a cluster phase for a confined finite chain of ions
Employing Monte-Carlo simulation techniques we investigate the statistical
properties of equally charged particles confined in a one-dimensional box trap
and detect a crossover from a crystalline to a cluster phase with increasing
temperature. The corresponding transition temperature depends separately on the
number of particles N and the box size L, implying non-extensivity due to the
long-range character of the interactions. The probability density of the
spacing between the particles exhibits at low temperatures an accumulation of
discrete peaks with an overall asymmetric shape. In the vicinity of the
transition temperature it is of a Gaussian form whereas in the high temperature
regime an exponential decay is observed. The high temperature behaviour shows a
cluster phase with a mean cluster size that first increases with the
temperature and then saturates. The crossover is clearly identifiable also in
the non-linear behaviour of the heat capacity with varying temperature. The
influence of the trapping potential on the observed results as well as possible
experimental realizations are briefly addressed.Comment: 12 pages, 13 figure
Reionization and Cosmology with 21 cm Fluctuations
Measurement of the spatial distribution of neutral hydrogen via the
redshifted 21 cm line promises to revolutionize our knowledge of the epoch of
reionization and the first galaxies, and may provide a powerful new tool for
observational cosmology from redshifts 1<z<4 . In this review we discuss recent
advances in our theoretical understanding of the epoch of reionization (EoR),
the application of 21 cm tomography to cosmology and measurements of the dark
energy equation of state after reionization, and the instrumentation and
observational techniques shared by 21 cm EoR and post reionization cosmology
machines. We place particular emphasis on the expected signal and observational
capabilities of first generation 21 cm fluctuation instruments.Comment: Invited review for Annual Review of Astronomy and Astrophysics (2010
volume
Electronic Structure Shift of Deep Nanoscale Silicon by SiO- vs. SiN-Embedding as Alternative to Impurity Doping
Conventional impurity doping of deep nanoscale silicon (dns-Si) used in ultra
large scale integration (ULSI) faces serious challenges below the 14 nm
technology node. We report on a new fundamental effect in theory and
experiment, namely the electronic structure of dns-Si experiencing energy
offsets of ca. 1 eV as a function of SiO- vs. SiN-embedding with a
few monolayers (MLs). An interface charge transfer (ICT) from dns-Si specific
to the anion type of the dielectric is at the core of this effect and arguably
nested in quantum-chemical properties of oxygen (O) and nitrogen (N) vs. Si. We
investigate the size up to which this energy offset defines the electronic
structure of dns-Si by density functional theory (DFT), considering interface
orientation, embedding layer thickness, and approximants featuring two Si
nanocrystals (NCs); one embedded in SiO and the other in SiN.
Working with synchrotron ultraviolet photoelectron spectroscopy (UPS), we use
SiO- vs. SiN-embedded Si nanowells (NWells) to obtain their energy
of the top valence band states. These results confirm our theoretical findings
and gauge an analytic model for projecting maximum dns-Si sizes for NCs,
nanowires (NWires) and NWells where the energy offset reaches full scale,
yielding to a clear preference for electrons or holes as majority carriers in
dns-Si. Our findings can replace impurity doping for n/p-type dns-Si as used in
ultra-low power electronics and ULSI, eliminating dopant-related issues such as
inelastic carrier scattering, thermal ionization, clustering, out-diffusion and
defect generation. As far as majority carrier preference is concerned, the
elimination of those issues effectively shifts the lower size limit of Si-based
ULSI devices to the crystalization limit of Si of ca. 1.5 nm and enables them
to work also under cryogenic conditions.Comment: 14 pages, 17 Figures with a total 44 graph
Fuzzy Set Methods for Object Recognition in Space Applications
Progress on the following four tasks is described: (1) fuzzy set based decision methodologies; (2) membership calculation; (3) clustering methods (including derivation of pose estimation parameters), and (4) acquisition of images and testing of algorithms
Semi-analytic Simulations of Galactic Winds: Volume Filling Factor, Ejection of Metals and Parameter Study
We present a semi-analytic treatment of galactic winds within high
resolution, large scale cosmological N-body simulations of a LCDM Universe. The
evolution of winds is investigated by following the expansion of supernova
driven superbubbles around the several hundred thousand galaxies that form in
an approximately spherical region of space with diameter 52 Mpc/h and mean
density close to the mean density of the Universe. We focus our attention on
the impact of winds on the diffuse intergalactic medium. Initial conditions for
mass loss at the base of winds are taken from Shu, Mo and Mao (2003). Results
are presented for the volume filling factor and the mass fraction of the IGM
affected by winds and their dependence on the model parameters is carefully
investigated. The mass loading efficiency of bubbles is a key factor to
determine the evolution of winds and their global impact on the IGM: the higher
the mass loading, the later the IGM is enriched with metals. Galaxies with 10^9
< M_* < 10^10 M_sun are responsible for most of the metals ejected into the IGM
at z=3, while galaxies with M_* < 10^9 M_sun give a non negligible contribution
only at higher redshifts, when larger galaxies have not yet assembled. We find
a higher mean IGM metallicity than Lyalpha forest observations suggest and we
argue that the discrepancy may be explained by the high temperatures of a large
fraction of the metals in winds, which may not leave detectable imprints in
absorption in the Lyalpha forest.Comment: 18 pages, 15 figures. Major changes in the model. Manuscript with
high resolution figures available upon request. MNRAS in pres
DeepSphere: Efficient spherical Convolutional Neural Network with HEALPix sampling for cosmological applications
Convolutional Neural Networks (CNNs) are a cornerstone of the Deep Learning
toolbox and have led to many breakthroughs in Artificial Intelligence. These
networks have mostly been developed for regular Euclidean domains such as those
supporting images, audio, or video. Because of their success, CNN-based methods
are becoming increasingly popular in Cosmology. Cosmological data often comes
as spherical maps, which make the use of the traditional CNNs more complicated.
The commonly used pixelization scheme for spherical maps is the Hierarchical
Equal Area isoLatitude Pixelisation (HEALPix). We present a spherical CNN for
analysis of full and partial HEALPix maps, which we call DeepSphere. The
spherical CNN is constructed by representing the sphere as a graph. Graphs are
versatile data structures that can act as a discrete representation of a
continuous manifold. Using the graph-based representation, we define many of
the standard CNN operations, such as convolution and pooling. With filters
restricted to being radial, our convolutions are equivariant to rotation on the
sphere, and DeepSphere can be made invariant or equivariant to rotation. This
way, DeepSphere is a special case of a graph CNN, tailored to the HEALPix
sampling of the sphere. This approach is computationally more efficient than
using spherical harmonics to perform convolutions. We demonstrate the method on
a classification problem of weak lensing mass maps from two cosmological models
and compare the performance of the CNN with that of two baseline classifiers.
The results show that the performance of DeepSphere is always superior or equal
to both of these baselines. For high noise levels and for data covering only a
smaller fraction of the sphere, DeepSphere achieves typically 10% better
classification accuracy than those baselines. Finally, we show how learned
filters can be visualized to introspect the neural network.Comment: arXiv admin note: text overlap with arXiv:astro-ph/0409513 by other
author
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