1,473 research outputs found
Accurate estimators of power spectra in N-body simulations
abridged] A method to rapidly estimate the Fourier power spectrum of a point
distribution is presented. This method relies on a Taylor expansion of the
trigonometric functions. It yields the Fourier modes from a number of FFTs,
which is controlled by the order N of the expansion and by the dimension D of
the system. In three dimensions, for the practical value N=3, the number of
FFTs required is 20. We apply the method to the measurement of the power
spectrum of a periodic point distribution that is a local Poisson realization
of an underlying stationary field. We derive explicit analytic expression for
the spectrum, which allows us to quantify--and correct for--the biases induced
by discreteness and by the truncation of the Taylor expansion, and to bound the
unknown effects of aliasing of the power spectrum. We show that these aliasing
effects decrease rapidly with the order N. The only remaining significant
source of errors is reduced to the unavoidable cosmic/sample variance due to
the finite size of the sample. The analytical calculations are successfully
checked against a cosmological N-body experiment. We also consider the initial
conditions of this simulation, which correspond to a perturbed grid. This
allows us to test a case where the local Poisson assumption is incorrect. Even
in that extreme situation, the third-order Fourier-Taylor estimator behaves
well. We also show how to reach arbitrarily large dynamic range in Fourier
space (i.e., high wavenumber), while keeping statistical errors in control, by
appropriately "folding" the particle distribution.Comment: 18 Pages, 9 Figures. Accepted for publication in MNRAS. The
Fourier-Taylor module as well as the associated power spectrum estimator tool
we propose is available as an F90 package, POWMES, at
http://www.projet-horizon.fr or on request from the author
Dust emission and star formation toward a redshift 5.5 QSO
We report observations of the low-luminosity z = 5.50 quasar RD
J030117+002025 (RD0301 hereafter) at 250 GHz (1.20mm) using the Max-Planck
Millimeter Bolometer (MAMBO) array at the IRAM 30-meter telescope. The quasar
was detected with a 1.2mm flux density of 0.87 +-0.20 mJy. The lack of
detectable 1.4 GHz radio emission indicates that the millimeter emission is of
thermal nature, making RD0301 the most distant dust-emission source known. When
matching a 50K grey body thermal far-infrared (FIR) spectrum to the observed
millimeter flux we imply a FIR luminosity ~4x10^12 L_sun, which is comparable
to the quasar's optical luminosity. If the FIR luminosity arises from massive
star formation, the implied star formation rate would be ~600 M_sun/yr,
comparable to that of the starburst galaxies which dominate the average star
formation and FIR emission in the early Universe. The FIR luminosity of RD0301
is close to the average of that found in optically far more luminous
high-redshift quasars. The comparably high millimeter to optical brightness
ratio of RD0301 is further evidence for that there is no strong correlation
between the optical and millimeter brightness of high-redshift quasars,
supporting the idea that in high-redshift quasars the dust is not heated by the
AGN, but by starbursts.Comment: 4 pages, 2 figures, accepted by Astronomy & Astrophysics Letter
On the luminosity distance and the epoch of acceleration
Standard cosmological models based on general relativity (GR) with dark
energy predict that the Universe underwent a transition from decelerating to
accelerating expansion at a moderate redshift . Clearly, it
is of great interest to directly measure this transition in a model-independent
way, without the assumption that GR is the correct theory of gravity. We
explore to what extent supernova (SN) luminosity distance measurements provide
evidence for such a transition: we show that, contrary to intuition, the
well-known "turnover" in the SN distance residuals relative to an
empty (Milne) model does not give firm evidence for such a transition within
the redshift range spanned by SN data. The observed turnover in that diagram is
predominantly due to the negative curvature in the Milne model, {\em not} the
deceleration predicted by CDM and relatives. We show that there are
several advantages in plotting distance residuals against a flat,
non-accelerating model , and also remapping the axis to ; we outline a number of useful and intuitive properties of this
presentation. We conclude that there are significant complementarities between
SNe and baryon acoustic oscillations (BAOs): SNe offer high precision at low
redshifts and give good constraints on the net {\em amount} of acceleration
since , but are weak at constraining ; while radial BAO
measurements are probably superior for placing direct constraints on .Comment: Latex, 13 pages, 7 figures. Accepted by MNRAS. For the busy reader,
Figs 4 and 6 are the main result
The interdisciplinary use of “overpressure”
Overpressure is a polysemic word that has a variety of meanings within and across different disciplines. This is likely to be a particular problem in analysis of geothermal resources, where reservoir engineers, volcanologists and structural geologists may each confidently use overpressure but mean different things. We suggest that, to avoid confusion, the term should be carefully and accurately defined whenever used, and ideally only used to mean fluid pressure in excess of hydrostatic pressure
Pan-genomic perspective on the evolution of the Staphylococcus aureus USA300 epidemic.
Staphylococcus aureus USA300 represents the dominant community-associated methicillin-resistant S. aureus lineage in the USA, where it is a major cause of skin and soft tissue infections. Previous comparative genomic studies have described the population structure and evolution of USA300 based on geographically restricted isolate collections. Here, we investigated the USA300 population by sequencing genomes of a geographically distributed panel of 191 clinical S. aureus isolates belonging to clonal complex 8 (CC8), derived from the Tigecycline Evaluation and Surveillance Trial program. Isolates were collected at 12 healthcare centres across nine USA states in 2004, 2009 or 2010. Reconstruction of evolutionary relationships revealed that CC8 was dominated by USA300 isolates (154/191, 81 %), which were heterogeneous and demonstrated limited phylogeographic clustering. Analysis of the USA300 core genomes revealed an increase in median pairwise SNP distance from 62 to 98 between 2004 and 2010, with a stable pattern of above average dN/dS ratios. The phylogeny of the USA300 population indicated that early diversification events led to the formation of nested clades, which arose through cumulative acquisition of predominantly non-synonymous SNPs in various coding sequences. The accessory genome of USA300 was largely homogenous and consisted of elements previously associated with this lineage. We observed an emergence of SCCmec negative and ACME negative USA300 isolates amongst more recent samples, and an increase in the prevalence of ϕSa5 prophage. Together, the analysed S. aureus USA300 collection revealed an evolving pan-genome through increased core genome heterogeneity and temporal variation in the frequency of certain accessory elements
Principal Component Analysis of Weak Lensing Surveys
We study degeneracies between cosmological parameters and measurement errors
from cosmic shear surveys using a principal component analysis of the Fisher
matrix. We simulate realistic survey topologies with non-uniform sky coverage,
and quantify the effect of survey geometry, depth and noise from intrinsic
galaxy ellipticities on the parameter errors. This analysis allows us to
optimise the survey geometry. Using the shear two-point correlation functions
and the aperture mass dispersion, we study various degeneracy directions in a
multi-dimensional parameter space spanned by Omega_m, Omega_Lambda, sigma_8,
the shape parameter Gamma, the spectral index n_s, along with parameters that
specify the distribution of source galaxies. If only three parameters are to be
obtained from weak lensing data, a single principal component is dominant and
contains all information about the main parameter degeneracies and their
errors. The variance of the dominant principal component of the Fisher matrix
shows a minimum for survey strategies which have small cosmic variance and
measure the shear correlation up to several degrees [abridged].Comment: 13 pages, 17 figures. A&A in press, matches the version to be
publishe
Solid-phase phosphorus speciation in Saharan Bodélé depression dusts and source sediments
Phosphorus (P) is one of the most important limiting nutrients for the growth of oceanic phytoplankton and terrestrial ecosystems, which in turn contributes to CO2 sequestration. The solid-phase speciation of P will influence its solubility and hence its availability to such ecosystems. This study reports on the results of X-ray diffraction, electron microprobe chemical analysis and X-ray mapping, chemical extractions and X-ray absorption near-edge spectroscopy analysis carried out to determine the solid-phase speciation of P in dusts and their source sediments from the Saharan Bodélé Depression, the world’s largest single source of dust. Chemical extraction data suggest that the Bodélé dusts contain 28 to 60% (mean 49%) P sorbed to, or co-precipitated with Fe (hydr)oxides, < 10% organic P, 21-50% (mean 32%) detrital apatite P, and 10-22% (mean 15%) authigenic-biogenic apatite P. This is confirmed by the other analyses, which also suggest that the authigenic-biogenic apatite P is likely fish bone and scale, and that this might form a larger proportion of the apatite pool (33 +/− 22%) than given by the extraction data. This is the first-ever report of fish material in aeolian dust, and it is significant because P derived from fish bone and scale is relatively soluble and is often used as a soil fertilizer. Therefore, the fish-P will likely be the most readily form of Bodélé P consumed during soil weathering and atmospheric processing, but given time and acid dissolution, the detrital apatite, Fe-P and organic-P will also be made available. The Bodélé dust input of P to global ecosystems will only have a limited life, however, because its major source materials, diatomite in the Bodélé Depression, undergo persistent deflation and have a finite thickness
Geophysical tests for habitability in ice-covered ocean worlds
Geophysical measurements can reveal the structure of icy ocean worlds and
cycling of volatiles. The associated density, temperature, sound speed, and
electrical conductivity of such worlds thus characterizes their habitability.
To explore the variability and correlation of these parameters, and to provide
tools for planning and data analyses, we develop 1-D calculations of internal
structure, which use available constraints on the thermodynamics of aqueous
MgSO, NaCl (as seawater), and NH, water ices, and silicate content.
Limits in available thermodynamic data narrow the parameter space that can be
explored: insufficient coverage in pressure, temperature, and composition for
end-member salinities of MgSO and NaCl, and for relevant water ices; and a
dearth of suitable data for aqueous mixtures of Na-Mg-Cl-SO-NH. For
Europa, ocean compositions that are oxidized and dominated by MgSO, vs
reduced (NaCl), illustrate these gaps, but also show the potential for
diagnostic and measurable combinations of geophysical parameters. The
low-density rocky core of Enceladus may comprise hydrated minerals, or anydrous
minerals with high porosity comparable to Earth's upper mantle. Titan's ocean
must be dense, but not necessarily saline, as previously noted, and may have
little or no high-pressure ice at its base. Ganymede's silicious interior is
deepest among all known ocean worlds, and may contain multiple phases of
high-pressure ice, which will become buoyant if the ocean is sufficiently
salty. Callisto's likely near-eutectic ocean cannot be adequately modeled using
available data. Callisto may also lack high-pressure ices, but this cannot be
confirmed due to uncertainty in its moment of inertia
Population genetics of trypanosoma brucei rhodesiense: clonality and diversity within and between foci
African trypanosomes are unusual among pathogenic protozoa in that they can undergo their complete morphological life cycle in the tsetse fly vector with mating as a non-obligatory part of this development. Trypanosoma brucei rhodesiense, which infects humans and livestock in East and Southern Africa, has classically been described as a host-range variant of the non-human infective Trypanosoma brucei that occurs as stable clonal lineages. We have examined T. b. rhodesiense populations from East (Uganda) and Southern (Malawi) Africa using a panel of microsatellite markers, incorporating both spatial and temporal analyses. Our data demonstrate that Ugandan T. b. rhodesiense existed as clonal populations, with a small number of highly related genotypes and substantial linkage disequilibrium between pairs of loci. However, these populations were not stable as the dominant genotypes changed and the genetic diversity also reduced over time. Thus these populations do not conform to one of the criteria for strict clonality, namely stability of predominant genotypes over time, and our results show that, in a period in the mid 1990s, the previously predominant genotypes were not detected but were replaced by a novel clonal population with limited genetic relationship to the original population present between 1970 and 1990. In contrast, the Malawi T. b. rhodesiense population demonstrated significantly greater diversity and evidence for frequent genetic exchange. Therefore, the population genetics of T. b. rhodesiense is more complex than previously described. This has important implications for the spread of the single copy T. b. rhodesiense gene that allows human infectivity, and therefore the epidemiology of the human disease, as well as suggesting that these parasites represent an important organism to study the influence of optional recombination upon population genetic dynamics
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