4,323 research outputs found
The Cluster Abundance in Flat and Open Cosmologies
We use the galaxy cluster X-ray temperature distribution function to
constrain the amplitude of the power spectrum of density inhomogeneities on the
scale corresponding to clusters. We carry out the analysis for critical density
universes, for low density universes with a cosmological constant included to
restore spatial flatness and for genuinely open universes. That clusters with
the same present temperature but different formation times have different
virial masses is included. We model cluster mergers using two completely
different approaches, and show that the final results from each are extremely
similar. We give careful consideration to the uncertainties involved, carrying
out a Monte Carlo analysis to determine the cumulative errors. For critical
density our result agrees with previous papers, but we believe the result
carries a larger uncertainty. For low density universes, either flat or open,
the required amplitude of the power spectrum increases as the density is
decreased. If all the dark matter is taken to be cold, then the cluster
abundance constraint remains compatible with both galaxy correlation data and
the {\it COBE} measurement of microwave background anisotropies for any
reasonable density.Comment: Uuencoded package containing LaTeX file (uses mn.sty) plus 7
postscript figures incorporated using epsf. Total length 10 pages. Final
version, to appear MNRAS. COBE comparison changed to 4yr data. No change to
results or conclusion
Cold dark matter models with high baryon content
Recent results have suggested that the density of baryons in the Universe,
OmegaB, is much more uncertain than previously thought, and may be
significantly higher. We demonstrate that a higher OmegaB increases the
viability of critical-density cold dark matter (CDM) models. High baryon
fraction offers the twin benefits of boosting the first peak in the microwave
anisotropy power spectrum and of suppressing short-scale power in the matter
power spectrum. These enable viable CDM models to have a larger Hubble constant
than otherwise possible. We carry out a general exploration of high OmegaB CDM
models, varying the Hubble constant h and the spectral index n. We confront a
variety of observational constraints and discuss specific predictions. Although
some observational evidence may favour baryon fractions as high as 20 per cent,
we find that values around 10 to 15 per cent provide a reasonable fit to a wide
range of data. We suggest that models with OmegaB in this range, with h about
0.5 and n about 0.8, are currently the best critical-density CDM models.Comment: 14 pages, LaTeX, with 9 included figures, to appear in MNRAS. Revised
version includes updated references, some changes to section 4. Conclusions
unchange
Process transparency on construction sites : examples from construction companies in Brazil
Process transparency is the core concept in Visual Management (VM), which is one of the founding blocks of the Toyota Production System. This paper presents the
preliminary results of a collaborative research conducted between Brazil and the UK, as part of a research effort focused on the application of Visual Management in
construction. How process transparency is realized on construction sites is the main research question of the paper. The use of this concept and the implementation of the
transparency theory were investigated through multiple case studies, carried out in nine different construction companies. The findings are explained through six theoretical transparency increasing approaches. The affecting parameters in the application of, the management’s perception of and several methods in process
transparency in construction were identified. Further work, especially exploring the functions of process transparency on construction sites and reflecting the worker perception of the issue, is necessary to elaborate the process transparency concept
Early Enrichment of the Intergalactic Medium and its Feedback on Galaxy Formation
Supernova-driven outflows from early galaxies may have had a large impact on
the kinetic and chemical structure of the intergalactic medium (IGM). We use
three-dimensional Monte Carlo cosmological realizations of a simple linear
peaks model to track the time evolution of such metal-enriched outflows and
their feedback on galaxy formation. We find that at most 30% of the IGM by
volume is enriched to values above 10^-3 solar in models that only include
objects that cool by atomic transitions. The majority of enrichment occurs
relatively early (5 < z < 12) and resulting in a mass-averaged cosmological
metallicity between 10^-3 and 10^-1.5 solar. The inclusion of Population III
objects that cool through H2 line emission has only a minor impact on these
results: increasing the mean metallicity and filling factor by at most a factor
of 1.4, and moving the dawn of the enrichment epoch to a redshift of
approximately 14 at the earliest. Thus enrichment by outflowing galaxies is
likely to have been incomplete and inhomogeneous, biased to the areas near the
starbursting galaxies themselves. Models with a 10% star formation efficiency
can satisfactorily reproduce the nearly constant (2 < z < 5, Z approximately
3.5 x 10^-4 solar) metallicity of the low column density Ly-alpha forest
derived by Songaila (2001), an effect of the decreasing efficiency of metal
loss from larger galaxies. Finally, we show that IGM enrichment is intimately
tied to the ram-pressure stripping of baryons from neighboring perturbations.
This results in the suppression of at least 20% of the dwarf galaxies in the
mass range 10^8.5 to 10^9.5 solar, in all models with filling factors greater
than 2%, and an overall suppression of approximately 50% of dwarf galaxies in
the most observationally-favored model.Comment: 8 pages, 5 figures, accepted to Ap
Cosmological Implications of the Fundamental Relations of X-ray Clusters
Based on the two-parameter family nature of X-ray clusters of galaxies
obtained in a separate paper, we discuss the formation history of clusters and
cosmological parameters of the universe. Utilizing the spherical collapse model
of cluster formation, and assuming that the cluster X-ray core radius is
proportional to the virial radius at the time of the cluster collapse, the
observed relations among the density, radius, and temperature of clusters imply
that cluster formation occurs in a wide range of redshift. The observed
relations favor the low-density universe. Moreover, we find that the model of
is preferable.Comment: 7 pages, 4 figures. To be published in ApJ Letter
Apparent and actual galaxy cluster temperatures
The redshift evolution of the galaxy cluster temperature function is a
powerful probe of cosmology. However, its determination requires the
measurement of redshifts for all clusters in a catalogue, which is likely to
prove challenging for large catalogues expected from XMM--Newton, which may
contain of order 2000 clusters with measurable temperatures distributed around
the sky. In this paper we study the apparent cluster temperature, which can be
obtained without cluster redshifts. We show that the apparent temperature
function itself is of limited use in constraining cosmology, and so concentrate
our focus on studying how apparent temperatures can be combined with other
X-ray information to constrain the redshift. We also briefly study the
circumstances in which non-thermal spectral features can give redshift
information.Comment: 7 pages LaTeX file with 13 figures incorporated (uses mn.sty and
epsf). Minor changes to match MNRAS accepted versio
The effects of subcurative praziquantel treatment on life-history traits and trade-offs in drug-resistant Schistosoma mansoni
Natural selection acts on all organisms, including parasites, to maximise reproductive fitness. Drug resistance traits are often associated with life-history costs in the absence of treatment. Schistosomiasis control programmes rely on mass drug administration to reduce human morbidity and mortality. Although hotspots of reduced drug efficacy have been reported, resistance is not widespread. Using Bayesian State-Space Models (SSMs) fitted to data from an in vivo laboratory system, we tested the hypothesis that the spread of resistant Schistosoma may be limited by life-history costs not present in susceptible counterparts. Schistosoma mansoni parasites from a praziquantel–susceptible (S), a praziquantel–resistant (R) or a mixed line of originally resistant and susceptible parasites (RS) were exposed to a range of praziquantel doses. Parasite numbers at each life stage were quantified in their molluscan intermediate and murine definitive hosts across four generations, and SSMs were used to estimate key life-history parameters for each experimental group over time. Model outputs illustrated that parasite adult survival and fecundity in the murine host decreased across all lines, including R, with increasing drug pressure. Trade-offs between adult survival and fecundity were observed in all untreated lines, and these remained strong in S with praziquantel pressure. In contrast, trade-offs between adult survival and fecundity were lost under praziquantel pressure in R. As expected, parasite life-history traits within the molluscan host were complex, but trade-offs were demonstrated between parasite establishment and cercarial output. The observed trade-offs between generations within hosts, which were modified by praziquantel treatment in the R line, could limit the spread of R parasites under praziquantel pressure. Whilst such complex life-history costs may be difficult to detect using standard empirical methods, we demonstrate that SSMs provide robust estimates of life history parameters, aiding our understanding of costs and trade-offs of resistant parasites within this system and beyond
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