7,383 research outputs found
Dynamics of merging: Post-merger mixing and relaxation of an Illustris galaxy
During the merger of two galaxies, the resulting system undergoes violent
relaxation and seeks stable equilibrium. However, the details of this evolution
are not fully understood. Using Illustris simulation, we probe two physically
related processes, mixing and relaxation. Though the two are driven by the same
dynamics---global time-varying potential for the energy, and torques caused by
asymmetries for angular momentum---we measure them differently. We define
mixing as the redistribution of energy and angular momentum between particles
of the two merging galaxies. We assess the degree of mixing as the difference
between the shapes of their N(E)s, and their N(L^2)s. We find that the
difference is decreasing with time, indicating mixing. To measure relaxation,
we compare N(E) of the newly merged system to N(E) of a theoretical prediction
for relaxed collisionless systems, DARKexp, and witness the system becoming
more relaxed, in the sense that N(E) approaches DARKexp N(E). Because the
dynamics driving mixing and relaxation are the same, the timescale is similar
for both. We measure two sequential timescales: a rapid, 1 Gyr phase after the
initial merger, during which the difference in N(E) of the two merging halos
decreases by ~80%, followed by a slow phase, when the difference decreases by
~50% over ~8.5 Gyrs. This is a direct measurement of the relaxation timescale.
Our work also draws attention to the fact that when a galaxy has reached Jeans
equilibrium it may not yet have reached a fully relaxed state given by DARKexp,
in that it retains information about its past history. This manifests itself
most strongly in stars being centrally concentrated. We argue that it is
particularly difficult for stars, and other tightly bound particles, to mix
because they have less time to be influenced by the fluctuating potential, even
across multiple merger events.Comment: accepted for publication in JCA
Ubiquity of density slope oscillations in the central regions of galaxy and cluster-sized systems
One usually thinks of a radial density profile as having a monotonically
changing logarithmic slope, such as in NFW or Einasto profiles. However, in two
different classes of commonly used systems, this is often not the case. These
classes exhibit non-monotonic changes in their density profile slopes which we
call oscillations for short. We analyze these two unrelated classes separately.
Class 1 consists of systems that have density oscillations and that are defined
through their distribution function , or differential energy distribution
, such as isothermal spheres, King profiles, or DARKexp, a theoretically
derived model for relaxed collisionless systems. Systems defined through
or generally have density slope oscillations. Class 1 system
oscillations can be found at small, intermediate, or large radii but we focus
on a limited set of Class 1 systems that have oscillations in the central
regions, usually at , where is the largest
radius where . We show that the shape of their
can roughly predict the amplitude of oscillations. Class 2 systems which are a
product of dynamical evolution, consist of observed and simulated galaxies and
clusters, and pure dark matter halos. Oscillations in the density profile slope
seem pervasive in the central regions of Class 2 systems. We argue that in
these systems, slope oscillations are an indication that a system is not fully
relaxed. We show that these oscillations can be reproduced by small
modifications to of DARKexp. These affect a small fraction of systems'
mass and are confined to . The size of these
modifications serves as a potential diagnostic for quantifying how far a system
is from being relaxed.Comment: accepted by the Journal of Cosmology and Astroparticle Physics (JCAP
Parallelization of a Six Degree of Freedom Entry Vehicle Trajectory Simulation Using OpenMP and OpenACC
The art and science of writing parallelized software, using methods such as Open Multi-Processing (OpenMP) and Open Accelerators (OpenACC), is dominated by computer scientists. Engineers and non-computer scientists looking to apply these techniques to their project applications face a steep learning curve, especially when looking to adapt their original single threaded software to run multi-threaded on graphics processing units (GPUs). There are significant changes in mindset that must occur; such as how to manage memory, the organization of instructions, and the use of if statements (also known as branching). The purpose of this work is twofold: 1) to demonstrate the applicability of parallelized coding methodologies, OpenMP and OpenACC, to tasks outside of the typical large scale matrix mathematics; and 2) to discuss, from an engineers perspective, the lessons learned from parallelizing software using these computer science techniques. This work applies OpenMP, on both multi-core central processing units (CPUs) and Intel Xeon Phi 7210, and OpenACC on GPUs. These parallelization techniques are used to tackle the simulation of thousands of entry vehicle trajectories through the integration of six degree of freedom (DoF) equations of motion (EoM). The forces and moments acting on the entry vehicle, and used by the EoM, are estimated using multiple models of varying levels of complexity. Several benchmark comparisons are made on the execution of six DoF trajectory simulation: single thread Intel Xeon E5-2670 CPU, multi-thread CPU using OpenMP, multi-thread Xeon Phi 7210 using OpenMP, and multi-thread NVIDIA Tesla K40 GPU using OpenACC. These benchmarks are run on the Pleiades Supercomputer Cluster at the National Aeronautics and Space Administration (NASA) Ames Research Center (ARC), and a Xeon Phi 7210 node at NASA Langley Research Center (LaRC)
Improved Smoothing Algorithms for Lattice Gauge Theory
The relative smoothing rates of various gauge field smoothing algorithms are
investigated on -improved \suthree Yang--Mills gauge field
configurations. In particular, an -improved version of APE
smearing is motivated by considerations of smeared link projection and cooling.
The extent to which the established benefits of improved cooling carry over to
improved smearing is critically examined. We consider representative gauge
field configurations generated with an -improved gauge field
action on \1 lattices at and \2 lattices at
having lattice spacings of 0.165(2) fm and 0.077(1) fm respectively. While the
merits of improved algorithms are clearly displayed for the coarse lattice
spacing, the fine lattice results put the various algorithms on a more equal
footing and allow a quantitative calibration of the smoothing rates for the
various algorithms. We find the relative rate of variation in the action may be
succinctly described in terms of simple calibration formulae which accurately
describe the relative smoothness of the gauge field configurations at a
microscopic level
The FLIC Overlap Quark Propagator
FLIC overlap fermions are a variant of the standard (Wilson) overlap action,
with the FLIC (Fat Link Irrelevant Clover) action as the overlap kernel rather
than the Wilson action. The structure of the FLIC overlap fermion propagator in
momentum space is studied, and a comparison against previous studies of the
Wilson overlap propagator in quenched QCD is performed. To explore the scaling
properties of the propagator for the two actions, numerical calculations are
performed in Landau Gauge across three lattices with different lattice spacing
and similar physical volumes. We find that at light quark masses the acti
ons agree in both the infrared and the ultraviolet, but at heavier masses some
disagreement in the ultraviolet appears. This is attributed to the two action s
having different discretisation errors with the FLIC overlap providing superior
performance in this regime. Both actions scale reasonably, but some scaling
violations are observed
Electrochemistry and application of a novel monosubstituted squarate electron-transfer mediator in a glucose oxidase-doped poly(phenol) sensor
Electrosynthetic poly(phenol) nanofilms were deposited in situ on platinum electrodes
in the presence and absence of glucose oxidase. The synthesis charges and currents of
the nonconducting polymer films were recorded at various applied potentials for films grown
from 25–100 mM phenol concentrations. Film parameters such as the standard rate constant
for film deposition, film thickness, and surface concentration of the poly(phenol) films were
evaluated from the cyclic and step voltammograms of the polymerization process. A novel
electron-transfer mediator consisting of monosubstituted 4-hydroxycyclobut-3-ene-1,2-dione
(squarate) was used as a mediator for Pt/poly(phenol) nano-film/GOx amperometric glucose
biosensors. Amperometric responses for 3-diphenylamino-4-hydroxycyclobut-3-ene-1,2-
dione (diphenylaminosquarate: E°′ = of +328 mV/Ag-AgCl at pH 7.0)-mediated systems
were measured by both steady-state amperometric and cyclic voltammetry. The sensor sensitivity
was calculated to be 558 nA cm
–2
(µM)
–1
Do Movement Patterns and Habitat Use Differ Between Optimal- and Suboptimal-sized Northern Bobwhite Coveys?
The group size of social animals and spatial structure of the environment can affect group behavior and movement decisions. Our objective was to investigate movement patterns and habitat use of northern bobwhite coveys (Colinus virginianus) of different size. Using radiotelemetry, we continuously monitored covey group size, daily movement, and habitat use on 12 independent 259-ha study areas in eastern Kansas, USA, during the winters between 1997 and 2000. We used correlated random walk models and fractal dimension models to determine if covey size affected movement characteristics or habitat selection. Intermediate-sized coveys (9–12 individuals, close to optimal covey size) exhibited daily movements that were substantially smaller and weekly home ranges that were more composed of woody escape cover than coveys of smaller or larger sizes. From the fractal dimension analyses, these coveys exhibited movement in between linear and a random walk at small spatial scales but very linear at large spatial scales. Large coveys had increased daily movement and tended to move in straighter lines (as indicated by the high proportion of turning angles [i.e., the angle between an initial direction and a new direction] around 0° and 180° and their multiscale fractal dimension) and they incorporated more cropland into their range, presumably to meet the feeding requirements of a larger group. In contrast, small coveys (1–4 individuals) tended to move more and increase the size of their home range, travel with a greater diversity of turning angles, and show movement patterns that were largely tortuous across a greater number of habitat patches at larger spatial scales (700 m). Small coveys have lower fitness and add new membership to increase fitness so it is possible that the movement behavior we observed represented a shift into a foray mode where bobwhites were searching for new membership. For areas with small populations and covey sizes, this information will help biologists better plan for habitat management to assist these coveys with their winter fitness
Gluons, quarks, and the transition from nonperturbative to perturbative QCD
Lattice-based investigations of two fundamental QCD quantities are described,
namely the gluon and quark propagators in Landau gauge. We have studied the
Landau gauge gluon propagator using a variety of lattices with spacings from a
= 0.17 to 0.41 fm. We demonstrate that it is possible to obtain scaling
behavior over a very wide range of momenta and lattice spacings and to explore
the infinite volume and continuum limits. These results confirm that the Landau
gauge gluon propagator is infrared finite. We study the Landau gauge quark
propagator in quenched QCD using two forms of the O(a)-improved propagator and
we find good agreement between these. The extracted value of the infrared quark
mass in the chiral limit is found to be 300 +/- 30 MeV. We conclude that the
momentum regime where the transition from nonperturbative to perturbative QCD
occurs is Q^2 approx 4GeV^2.Comment: 8 pages, 6 figures, 1 table. Talk presented by AGW at the Workshop on
Lepton Scattering, Hadrons and QCD, March 26-April 5, 2001, CSSM, Adelaide,
Australia. To appear in the proceeding
Kyawthuite, Bi^(3+)Sb^(5+)O_4, a new gem mineral from Mogok, Burma (Myanmar)
Kyawthuite, Bi^(3+)Sb^(5+)O_4, is a new gem mineral found as a waterworn crystal in alluvium at Chaung-gyi-ah-le-ywa in the Chaung-gyi valley, near Mogok, Burma (Myanmar). Its description is based upon a single sample, which was faceted into a 1.61-carat gem. The composition suggests that the mineral formed in a pegmatite. Kyawthuite is monoclinic, space group I2/c, with unit cell dimensions a = 5.4624(4), b = 4.88519(17), c = 11.8520(8) Å, β = 101.195(7)°, V = 310.25(3) Å^3 and Z = 4. The colour is reddish orange and the streak is white. It is transparent with adamantine lustre. The Mohs hardness is 5½. Kyawthuite is brittle with a conchoidal fracture and three cleavages: {001} perfect, {110} and {110} good. The measured density is 8.256(5) g cm^(–3) and the calculated density is 8.127 g cm^(–3). The mineral is optically biaxial with 2V = 90(2)°. The predicted indices of refraction are α = 2.194, β = 2.268, γ = 2.350. Pleochroism is imperceptible and the optical orientation is X = b; Y ≈ c; Z ≈ a. Electron microprobe analyses, provided the empirical formula (Bi^(3+)_(0.82)Sb^(3+)_(0.18))_(Σ1.00)( Sb^(5+)_(0.99)Ta^(5+)_(0.01))_(Σ1.00)O_4. The Raman spectrum is similar to that of synthetic Bi^(3+)Sb^(5+)O_4. The infrared spectrum shows a trace amount of OH/H_2O. The eight strongest powder X-ray diffraction lines are [d_(obs) in Å(I)(hkl)]: 3.266(100)(112), 2.900(66)(112), 2.678(24)(200), 2.437(22)(020, 14), 1.8663(21)(024), 1.8026(43)(16,220,204), 1.6264(23)(224,116) and 1.5288(28)(312, 32). In the crystal structure of kyawthuite (R_1 = 0.0269 for 593 reflections with F_o > 4σF), Sb^(5+)O_6 octahedra share corners to form chequerboard-like sheets parallel to {001}. Atoms of Bi^(3+), located above and below the open squares in the sheets, form bonds to the O atoms in the sheets, thereby linking adjacent sheets into a framework. The Bi^(3+) atom is in lopsided 8 coordination, typical of a cation with stereoactive lone electron pairs. Kyawthuite is isostructural with synthetic β-Sb_2O_4 and clinocervantite (natural β-Sb_2O_4)
Interpersonal violence in peacetime Malawi.
Background: The contribution of interpersonal violence (IPV) to trauma burden varies greatly by region. The high rates of IPV in sub-Saharan Africa are thought to relate in part to the high rates of collective violence. Malawi, a country with no history of internal collective violence, provides an excellent setting to evaluate whether collective violence drives the high rates of IPV in this region.
Methods: This is a retrospective review of a prospective trauma registry from 2009 through 2016 at Kamuzu Central Hospital in Lilongwe, Malawi. Adult (\u3e16 years) victims of IPV were compared with non-intentional trauma victims. Log binomial regression determined factors associated with increased risk of mortality for victims of IPV.
Results: Of 72 488 trauma patients, 25 008 (34.5%) suffered IPV. Victims of IPV were more often male (80.2% vs. 74.8%; p
Discussion: Even in a sub-Saharan country that never experienced internal collective violence, IPV injury rates are high. Public health efforts to measure and address alcohol use, and studies to determine the role of mob justice, poverty, and intimate partner violence in IPV, in Malawi are needed.
Level of evidence: Level III
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