852 research outputs found
Parallel HOP: A Scalable Halo Finder for Massive Cosmological Data Sets
Modern N-body cosmological simulations contain billions () of dark
matter particles. These simulations require hundreds to thousands of gigabytes
of memory, and employ hundreds to tens of thousands of processing cores on many
compute nodes. In order to study the distribution of dark matter in a
cosmological simulation, the dark matter halos must be identified using a halo
finder, which establishes the halo membership of every particle in the
simulation. The resources required for halo finding are similar to the
requirements for the simulation itself. In particular, simulations have become
too extensive to use commonly-employed halo finders, such that the
computational requirements to identify halos must now be spread across multiple
nodes and cores. Here we present a scalable-parallel halo finding method called
Parallel HOP for large-scale cosmological simulation data. Based on the halo
finder HOP, it utilizes MPI and domain decomposition to distribute the halo
finding workload across multiple compute nodes, enabling analysis of much
larger datasets than is possible with the strictly serial or previous parallel
implementations of HOP. We provide a reference implementation of this method as
a part of the toolkit yt, an analysis toolkit for Adaptive Mesh Refinement
(AMR) data that includes complementary analysis modules. Additionally, we
discuss a suite of benchmarks that demonstrate that this method scales well up
to several hundred tasks and datasets in excess of particles. The
Parallel HOP method and our implementation can be readily applied to any kind
of N-body simulation data and is therefore widely applicable.Comment: 29 pages, 11 figures, 2 table
Face-Specific Resting Functional Connectivity between the Fusiform Gyrus and Posterior Superior Temporal Sulcus
Faces activate specific brain regions in fMRI, including the fusiform gyrus (FG) and the posterior superior temporal sulcus (pSTS). The fact that the FG and pSTS are frequently co-activated suggests that they may interact synergistically in a distributed face processing network. Alternatively, the functions implemented by these regions may be encapsulated from each other. It has proven difficult to evaluate these two accounts during visual processing of face stimuli. However, if the FG and pSTS interact during face processing, the substrate for such interactions may be apparent in a correlation of the BOLD timeseries from these two regions during periods of rest when no faces are present. To examine face-specific resting correlations, we developed a new partial functional connectivity approach in which we removed variance from the FG that was shared with other category-selective and control regions. The remaining face-specific FG resting variance was then used to predict resting signals throughout the brain. In two experiments, we observed face-specific resting functional connectivity between FG and pSTS, and importantly, these correlations overlapped precisely with the face-specific pSTS region obtained from independent localizer runs. Additional region-of-interest and pattern analyses confirmed that the FG–pSTS resting correlations were face-specific. These findings support a model in which face processing is distributed among a finite number of connected, but nevertheless face-specialized regions. The discovery of category-specific interactions in the absence of visual input suggests that resting networks may provide a latent foundation for task processing
The Birth of a Galaxy - III. Propelling reionisation with the faintest galaxies
Starlight from galaxies plays a pivotal role throughout the process of cosmic
reionisation. We present the statistics of dwarf galaxy properties at z > 7 in
haloes with masses up to 10^9 solar masses, using a cosmological radiation
hydrodynamics simulation that follows their buildup starting with their
Population III progenitors. We find that metal-enriched star formation is not
restricted to atomic cooling ( K) haloes, but can occur
in haloes down to masses ~10^6 solar masses, especially in neutral regions.
Even though these smallest galaxies only host up to 10^4 solar masses of stars,
they provide nearly 30 per cent of the ionising photon budget. We find that the
galaxy luminosity function flattens above M_UV ~ -12 with a number density that
is unchanged at z < 10. The fraction of ionising radiation escaping into the
intergalactic medium is inversely dependent on halo mass, decreasing from 50 to
5 per cent in the mass range . Using our galaxy
statistics in a semi-analytic reionisation model, we find a Thomson scattering
optical depth consistent with the latest Planck results, while still being
consistent with the UV emissivity constraints provided by Ly forest
observations at z = 4-6.Comment: 21 pages, 15 figures, 4 tables. Accepted in MNRA
The Biermann Battery in Cosmological MHD Simulations of Population III Star Formation
We report the results of the first self-consistent three-dimensional adaptive
mesh refinement magnetohydrodynamical simulations of Population III star
formation including the Biermann Battery effect. We find that the Population
III stars formed including this effect are both qualitatively and
quantitatively similar to those from hydrodynamics-only (non-MHD) cosmological
simulations. We observe peak magnetic fields of ~10^-9 G in the center of our
star-forming halo at z ~ 17.55. The magnetic fields created by the Biermann
Battery effect are predominantly formed early in the evolution of the
primordial halo at low density and large spatial scales, and then grow through
compression and by shear flows. The fields seen in this calculation are never
large enough to be dynamically important (with beta >= 10^{15} at all times),
and should be considered the minimum possible fields in existence during
Population III star formation, and may be seed fields for the stellar dynamo or
the magnetorotational instability at higher densities and smaller spatial
scales.Comment: 4 pages, 3 figures; Submitted to the Astrophysical Journal Letters.
Comments welcome. Typo found (and fixed) in equation
Open questions in the study of population III star formation
The first stars were key drivers of early cosmic evolution. We review the
main physical elements of the current consensus view, positing that the first
stars were predominantly very massive. We continue with a discussion of
important open questions that confront the standard model. Among them are
uncertainties in the atomic and molecular physics of the hydrogen and helium
gas, the multiplicity of stars that form in minihalos, and the possible
existence of two separate modes of metal-free star formation.Comment: 15 pages, 2 figures. To appear in the conference proceedings for IAU
Symposium 255: Low-Metallicity Star Formation: From the First Stars to Dwarf
Galaxie
The Santa Fe Light Cone Simulation Project: II. The Prospects for Direct Detection of the WHIM with SZE Surveys
Detection of the Warm-Hot Intergalactic Medium (WHIM) using Sunyaev-Zeldovich
effect (SZE) surveys is an intriguing possibility, and one that may allow
observers to quantify the amount of "missing baryons" in the WHIM phase. We
estimate the necessary sensitivity for detecting low density WHIM gas with the
South Pole Telescope (SPT) and Planck Surveyor for a synthetic 100 square
degree sky survey. This survey is generated from a very large, high dynamic
range adaptive mesh refinement cosmological simulation performed with the Enzo
code. We find that for a modest increase in the SPT survey sensitivity (a
factor of 2-4), the WHIM gas makes a detectable contribution to the integrated
sky signal. For a Planck-like satellite, similar detections are possible with a
more significant increase in sensitivity (a factor of 8-10). We point out that
for the WHIM gas, the kinematic SZE signal can sometimes dominate the thermal
SZE where the thermal SZE decrement is maximal (150 GHz), and that using the
combination of the two increases the chance of WHIM detection using SZE
surveys. However, we find no evidence of unique features in the thermal SZE
angular power spectrum that may aid in its detection. Interestingly, there are
differences in the power spectrum of the kinematic SZE, which may not allow us
to detect the WHIM directly, but could be an important contaminant in
cosmological analyses of the kSZE-derived velocity field. Corrections derived
from numerical simulations may be necessary to account for this contamination.Comment: 9 pages, submitted to Astrophysical Journa
Evolution and Distribution of Magnetic Fields from AGNs in Galaxy Clusters. I. The Effect of Injection Energy and Redshift
We present a series of cosmological magnetohydrodynamic (MHD) simulations
that simultaneously follow the formation of a galaxy cluster and evolution of
magnetic fields ejected by an Active Galactic Nucleus (AGN). Specifically, we
investigate the influence of both the epoch of AGN (z 3-0.5) and the AGN
energy ( 3 10 - 2 10 ergs)on the final
magnetic field distribution in a relatively massive cluster (M
10 M). We find that as long as the AGN magnetic fields are
ejected before the major mergers in the cluster formation history, magnetic
fields can be transported throughout the cluster and can be further amplified
by the intra-cluster medium (ICM) turbulence cause by hierarchical mergers
during the cluster formation process. The total magnetic energy in the cluster
can reach ergs, with micro Gauss fields distributed over
Mpc scale. The amplification of the total magnetic energy by the ICM
turbulence can be significant, up to 1000 times in some cases. Therefore
even weak magnetic fields from AGNs can be used to magnetize the cluster to the
observed level. The final magnetic energy in the ICM is determined by the ICM
turbulent energy, with a weak dependence on the AGN injection energy. We
discuss the properties of magnetic fields throughout the cluster and the
synthetic Faraday rotation measure maps they produce. We also show that high
spatial resolution over most of the magnetic regions of the cluster is very
important to capture the small scale dynamo process and maintain the magnetic
field structure in our simulations.Comment: 38 pages, 18 figures, Accepted for publication in Ap
Formation Channels for Population III Stars at Cosmic Dawn
We present a study of the co-evolution of a population of primordial
star-forming minihalos at Cosmic Dawn. In this study, we highlight the
influence of individual Population III stars on the ability of nearby minihalos
to form sufficient molecular hydrogen to undergo star formation of their own.
In the absence of radiation, we find the minimum halo mass required to bring
about collapse and star formation to be 10^5 Msun, which then increases to 10^6
Msun after two stars have formed. We find an inverse relationship between the
mass of a halo and the time required for it to recover its molecular gas after
being disrupted by radiation from a nearby star. We also take advantage of the
extremely high resolution to investigate the effects of major and minor mergers
on the gas content of star-forming minihalos. Contrary to previous claims of
fallback of supernova ejecta, we find that minihalos evacuated after hosting
Pop III stars primarily recover gas through mergers with undisturbed halos. We
identify an intriguing type of major merger between recently evacuated halos
and gas-rich ones, finding that these "dry" mergers accelerate star formation
instead of suppressing it like their low redshift counterparts. We attribute
this to the gas-poor nature of one of the merging halos resulting in no
significant rise in temperature or turbulence and instead inducing a rapid
increase in central density and hydrostatic pressure. This constitutes a novel
formation pathway for Pop III stars and establishes major mergers as
potentially the primary source of gas, thus redefining the role of major
mergers at this epoch.Comment: 14 pages, 12 figures, submitted to MNRA
The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations
By definition, Population III stars are metal-free, and their protostellar
collapse is driven by molecular hydrogen cooling in the gas-phase, leading to
large characteristic masses. Population II stars with lower characteristic
masses form when the star-forming gas reaches a critical metallicity of 10^{-6}
- 10^{-3.5} Z_\odot. We present an adaptive mesh refinement radiation
hydrodynamics simulation that follows the transition from Population III to II
star formation. The maximum spatial resolution of 1 comoving parsec allows for
individual molecular clouds to be well-resolved and their stellar associations
to be studied in detail. We model stellar radiative feedback with adaptive ray
tracing. A top-heavy initial mass function for the Population III stars is
considered, resulting in a plausible distribution of pair-instability
supernovae and associated metal enrichment. We find that the gas fraction
recovers from 5 percent to nearly the cosmic fraction in halos with merger
histories rich in halos above 10^7 solar masses. A single pair-instability
supernova is sufficient to enrich the host halo to a metallicity floor of
10^{-3} Z_\odot and to transition to Population II star formation. This
provides a natural explanation for the observed floor on damped Lyman alpha
(DLA) systems metallicities reported in the literature, which is of this order.
We find that stellar metallicities do not necessarily trace stellar ages, as
mergers of halos with established stellar populations can create superpositions
of t-Z evolutionary tracks. A bimodal metallicity distribution is created after
a starburst occurs when the halo can cool efficiently through atomic line
cooling.Comment: 11 pages, 7 figures; replaced with accepted version to ApJ;
additional movies and images can be found at
http://www.astro.princeton.edu/~jwise/research/GalaxyBirth.htm
Recommended from our members
Neurocognitive therapeutics: from concept to application in the treatment of negative attention bias
There is growing interest in the use of neuroimaging for the direct treatment of mental illness. Here, we present a new framework for such treatment, neurocognitive therapeutics. What distinguishes neurocognitive therapeutics from prior approaches is the use of precise brain-decoding techniques within a real-time feedback system, in order to adapt treatment online and tailor feedback to individuals’ needs. We report an initial feasibility study that uses this framework to alter negative attention bias in a small number of patients experiencing significant mood symptoms. The results are consistent with the promise of neurocognitive therapeutics to improve mood symptoms and alter brain networks mediating attentional control. Future work should focus on optimizing the approach, validating its effectiveness, and expanding the scope of targeted disorders
- …