4,268 research outputs found
Retour vers le futur : le commerce dâĂ©change international au cours des annĂ©es 80
Barter has been commonly portrayed as a dangerous element in international relations. Barter is portrayed in much of the literature as a cyclical phenomenon which flourishes with economic depression and fades away with prosperity. It is also said by many critics that barter should be considered a destructive economic practice. Finally, great emphasis is placed on the geopolitical importance of barter. In particular, barter is associated with the aggressive economic statecraft practiced by Nazi Germany in the 1930s.This article attempts to look more closely at the assumptions of the critics of modem barter. The author concludes that there are fundamental differences between the 1930s and the 1980s. Barter in the 1980s appears not to point towards a destructive past but a dynamic and complex future
Spatially Resolved Galaxy Star Formation and its Environmental Dependence I
We use the photometric information contained in individual pixels of 44,964
(0.019<z<0.125 and -23.5<M_r<-20.5) galaxies in the Fourth Data Release (DR4)
of the Sloan Digital Sky Survey to investigate the effects of environment on
galaxy star formation (SF). We use the pixel-z technique, which combines
stellar population synthesis models with photometric redshift template fitting
on the scale of individual pixels in galaxy images. Spectral energy
distributions are constructed, sampling a wide range of properties such as age,
star formation rate (SFR), dust obscuration and metallicity. By summing the
SFRs in the pixels, we demonstrate that the distribution of total galaxy SFR
shifts to lower values as the local density of surrounding galaxies increases,
as found in other studies. The effect is most prominent in the galaxies with
the highest star formation, and we see the break in the SFR-density relation at
a local galaxy density of (Mpc/h). Since our method
allows us to spatially resolve the SF distribution within galaxies, we can
calculate the mean SFR of each galaxy as a function of radius. We find that on
average the mean SFR is dominated by SF in the central regions of galaxies, and
that the trend for suppression of SFR in high density environments is driven by
a reduction in this nuclear SF. We also find that the mean SFR in the outskirts
is largely independent of environmental effects. This trend in the mean SFR is
shared by galaxies which are highly star forming, while those which are weakly
star forming show no statistically significant correlation between their
environment and the mean SFR at any radius.Comment: 37 pages, 11 figures. Referee's comments included and matches version
accepted for publication in the Astrophysical Journal. For high resolution
figures, see http://www.phyast.pitt.edu/~welikala/pixelz/paper1
The application of inelastic neutron scattering to investigate the interaction of methyl propanoate with silica
A modern industrial route for the manufacture of methyl methacrylate involves the reaction of methyl propanoate and formaldehyde over a silica-supported Cs catalyst. Although the process has been successfully commercialised, little is known about the surface interactions responsible for the forward chemistry. This work concentrates upon the interaction of methyl propanoate over a representative silica. A combination of infrared spectroscopy, inelastic neutron scattering, DFT calculations, X-ray diffraction and temperature-programmed desorption is used to deduce how the ester interacts with the silica surface
Effect of random and hub gene disruptions on environmental and mutational robustness in Escherichia coli
BACKGROUND: Genome-wide profiling has allowed the regulatory interaction networks of many organisms to be visualised and the pattern of connections between genes to be studied. These networks are non-random, following a power-law distribution with a small number of well-connected 'hubs' and many genes with only one or a few connections. Theoretical work predicts that power-law networks display several unique properties. One of the most biologically interesting of these is an intrinsic robustness to disturbance such that removal of a random gene will have little effect on network function. Conversely, targeted removal of a hub gene is expected to have a large effect. RESULTS: We compared the response of Escherichia coli to environmental and mutational stress following disruption of random or hub genes. We found that disruption of random genes had less effect on robustness to environmental stress than did the targeted disruption of hub genes. In contrast, random disruption strains were slightly less robust to the effect of mutational stress than were hub disruption strains. When we compared the effect of each disruption on environmental and mutational stress, we found a negative relationship, such that strains that were more environmentally robust tended to be less robust to mutational stress. CONCLUSION: Our results demonstrate that mutant strains of E. coli respond differently to stress, depending on whether random or hub genes are disrupted. This difference indicates that the power-law distribution of regulatory interactions has biological significance, making random disruptions less deleterious to organisms facing environmental stress. That E. coli can reduce the effect of environmental stress without reducing the phenotypic effect of additional mutations, indicates that robustness and evolvability need not be antagonistic
A Predicted Correlation Between Age Gradient and Star Formation History in FIRE Dwarf Galaxies
We explore the radial variation of star formation histories in dwarf galaxies
simulated with Feedback In Realistic Environments (FIRE) physics. The sample
contains 9 low-mass field dwarfs with M_ star = 10^5 - 10^7 M_sun from previous
FIRE results, and a new suite of 17 higher mass field dwarfs with M_star = 10^7
- 10^9 M_sun introduced here. We find that age gradients are common in our
dwarfs, with older stars dominant at large radii. The strength of the gradient
correlates with overall galaxy age such that earlier star formation produces a
more pronounced gradient. The relation between formation time and strength of
the gradient is driven by both mergers and star-formation feedback. Mergers can
both steepen and flatten the age gradient depending on the timing of the merger
and star formation history of the merging galaxy. In galaxies without
significant mergers, early feedback pushes stars to the outskirts at early
times. Interestingly, among galaxies without mergers, those with large dark
matter cores have flatter age gradients because these galaxies have more
late-time feedback. If real galaxies have age gradients as we predict, stellar
population studies that rely on sampling a limited fraction of a galaxy can
give a biased view of its global star formation history. We show that central
fields can be biased young by a few Gyrs while outer fields are biased old.
Fields positioned near the 2D half-light radius will provide the least biased
measure of a dwarf galaxy's global star formation history.Comment: 13 pages, 8 figures. Submitted to MNRAS, comments welcom
A general approach to maximise information density in neutron reflectometry analysis
Neutron and X-ray reflectometry are powerful techniques facilitating the
study of the structure of interfacial materials. The analysis of these
techniques is ill-posed in nature requiring the application of a
model-dependent methods. This can lead to the over- and under- analysis of
experimental data, when too many or too few parameters are allowed to vary in
the model. In this work, we outline a robust and generic framework for the
determination of the set of free parameters that is capable of maximising the
in-formation density of the model. This framework involves the determination of
the Bayesian evidence for each permutation of free parameters; and is applied
to a simple phospholipid monolayer. We believe this framework should become an
important component in reflectometry data analysis, and hope others more
regularly consider the relative evidence for their analytical models
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