15,833 research outputs found
The behaviour of shape and velocity anisotropy in dark matter haloes
Dark matter haloes from cosmological N-body simulations typically have
triaxial shapes and anisotropic velocity distributions. Recently it has been
shown that the velocity anisotropy, beta, of cosmological haloes and major
merger remnants depends on direction in such a way that beta is largest along
the major axis and smallest along the minor axis. In this work we use a wide
range of non-cosmological N-body simulations to examine halo shapes and
direction-dependence of velocity anisotropy profiles. For each of our simulated
haloes we define 48 cones pointing in different directions, and from the
particles inside each cone we compute velocity anisotropy profiles. We find
that elongated haloes can have very distinct velocity anisotropies. We group
the behaviour of haloes into three different categories, that range from
spherically symmetric profiles to a much more complex behaviour, where
significant differences are found for beta along the major and minor axes. We
encourage future studies of velocity anisotropies in haloes from cosmological
simulations to calculate beta-profiles in cones, since it reveals information,
which is hidden from a spherically averaged profile. Finally, we show that
spherically averaged profiles often obey a linear relation between beta and the
logarithmic density slope in the inner parts of haloes, but this relation is
not necessarily obeyed, when properties are calculated in cones.Comment: 23 pages, 14 figures. Accepted for publication in JCA
Observational constraints on the inflaton potential combined with flow-equations in inflaton space
Direct observations provide constraints on the first two derivatives of the
inflaton potential in slow roll models. We discuss how present day
observations, combined with the flow equations in slow roll parameter space,
provide a non-trivial constraint on the third derivative of the inflaton
potential. We find a lower bound on the third derivative of the inflaton
potential V'''/V > -0.2. We also show that unless the third derivative of the
inflaton potential is unreasonably large, then one predicts the tensor to
scalar ratio, r, to be bounded from below r > 3 * 10^{-6}.Comment: 4 pages, 2 figures. Important sign mistake corrected. Conclusions,
abstract and discussion change
Asymmetric velocity anisotropies in remnants of collisionless mergers
Dark matter haloes in cosmological N-body simulations are affected by
processes such as mergers, accretion and the gravitational interaction with
baryonic matter. Typically the analysis of dark matter haloes is performed in
spherical or elliptical bins and the velocity distributions are often assumed
to be constant within those bins. However, the velocity anisotropy, which
describes differences between the radial and tangential velocity dispersion,
has recently been show to have a strong dependence on direction in the triaxial
halos formed in cosmological simulations. In this study we derive properties of
particles in cones parallel or perpendicular to the collision axis of merger
remnants. We find that the velocity anisotropy has a strong dependence on
direction. The finding that the direction-dependence of the velocity anisotropy
of a halo depends on the merger history, explains the existence of such trends
in cosmological simulations. It also explains why a large diversity is seen in
the velocity anisotropy profiles in the outer parts of high-resolution
simulations of cosmological haloes.Comment: 19 pages, 15 figures, Resubmitted to JCAP after referee comment
Particle ejection during mergers of dark matter halos
Dark matter halos are built from accretion and merging. During merging some
of the dark matter particles may be ejected with velocities higher than the
escape velocity. We use both N-body simulations and single-particle
smooth-field simulations to demonstrate that rapid changes to the mean field
potential are responsible for such ejection, and in particular that dynamical
friction plays no significant role in it. Studying a range of minor mergers, we
find that typically between 5-15% of the particles from the smaller of the two
merging structures are ejected. We also find that the ejected particles
originate essentially from the small halo, and more specifically are particles
in the small halo which pass later through the region in which the merging
occurs.Comment: 18 pages, 12 figures. Accepted for publication in JCA
Thermodynamically self-consistent liquid state theories for systems with bounded potentials
The mean spherical approximation (MSA) can be solved semi-analytically for
the Gaussian core model (GCM) and yields - rather surprisingly - exactly the
same expressions for the energy and the virial equations. Taking advantage of
this semi-analytical framework, we apply the concept of the self-consistent
Ornstein-Zernike approximation (SCOZA) to the GCM: a state-dependent function K
is introduced in the MSA closure relation which is determined to enforce
thermodynamic consistency between the compressibility route and either the
virial or energy route. Utilizing standard thermodynamic relations this leads
to two different differential equations for the function K that have to be
solved numerically. Generalizing our concept we propose an
integro-differential-equation based formulation of the SCOZA which, although
requiring a fully numerical solution, has the advantage that it is no longer
restricted to the availability of an analytic solution for a particular system.
Rather it can be used for an arbitrary potential and even in combination with
other closure relations, such as a modification of the hypernetted chain
approximation.Comment: 11 pages, 11 figures, submitted to J. Chem. Phy
Current strategies for mobilome research
Mobile genetic elements (MGE) are pivotal for bacterial evolution and adaptation, allowing shuffling of genes even between distantly related bacterial species. The study of MGEs is biologically interesting as the mode of genetic propagation is kaleidoscopic and important, as MGEs are the main vehicles of the increasing bacterial antibiotic resistance that causes thousands of human deaths each year. The study of MGEs has previously focused on plasmids from individual isolates, but the revolution in sequencing technology has allowed the study of mobile genomic elements of entire communities using metagenomic approaches. The problem in using metagenomic sequencing for the study of MGEs is that plasmids and other mobile elements only comprise a small fraction of the total genetic content that are difficult to separate from chromosomal DNA based on sequence alone. Several different approaches have been proposed that specifically enrich plasmid DNA from community samples. Here, we review recent approaches used to study entire plasmid pools from complex environments, and point out possible future developments for and pitfalls of these approaches. Further, we discuss the use of the PacBio long-read sequencing technology for MGE discovery
Message passing for vertex covers
Constructing a minimal vertex cover of a graph can be seen as a prototype for
a combinatorial optimization problem under hard constraints. In this paper, we
develop and analyze message passing techniques, namely warning and survey
propagation, which serve as efficient heuristic algorithms for solving these
computational hard problems. We show also, how previously obtained results on
the typical-case behavior of vertex covers of random graphs can be recovered
starting from the message passing equations, and how they can be extended.Comment: 25 pages, 9 figures - version accepted for publication in PR
First-Pass Meconium Samples from Healthy Term Vaginally-Delivered Neonates : An Analysis of the Microbiota
Acknowledgments The authors would like to thank the parents who consented to provide samples with limited notice at an emotional and stressful time. This work was supported entirely from personal donations to the neonatal endowments fund at Aberdeen Maternity Hospital and we thank families for their continued generosity, year-on-year. The Rowett Institute of Nutrition and Health receives funding from the Scottish Government (SG-RESAS). Funding: This work was funded from NHS Grampian Neonatal Endowments. The Rowett Institute receives funding from the Rural and Environmental Science and Analytical Services programme of the Scottish Government. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Peer reviewedPublisher PD
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