3,644 research outputs found
Evolution of Compact Groups of Galaxies I. Merging Rates
We discuss the merging rates in compact groups of 5 identical elliptical
galaxies. All groups have the same mass and binding energy. We consider both
cases with individual halos and cases where the halo is common to all galaxies
and enveloping the whole group. In the latter situation the merging rate is
slower if the halo is more massive. The mass of individual halos has little
influence on the merging rates, due to the fact that all galaxies in our
simulations have the same mass, and so the more extended ones have a smaller
velocity dispersion. Groups with individual halos merge faster than groups with
common halos if the configuration is centrally concentrated, like a King
distribution of index 10. On the other hand for less concentrated
configurations the merging is initially faster for individual halo cases, and
slower after part of the group has merged. In cases with common halo, centrally
concentrated configurations merge faster for high halo-to-total mass ratios and
slower for low halo-to-total mass ratios. Groups whose virial ratio is
initially less than one merge faster, while groups that have initially
cylindrical rotation merge slower than groups starting in virial equilibrium.
In order to test how long a virialised group can survive before merging we
followed the evolution of a group with a high halo-to-total mass ratio and a
density distribution with very little central concentration. We find that the
first merging occurred only after a large number of crossing times, which with
areasonable calibration should be larger than a Hubble time. Hence, at least
for appropriate initial conditions, the longevity of compact groups is not
necessarily a problem, which is an alternative explanation to why we observe so
many compact groups despite the fact that their lifetimes seem short.Comment: 15 pages Latex, with 12 figures included, requires mn.sty, accepted
for publication in MNRA
Characteristics and performance of the ESTEC large space simulator cryogenic system
The final concept and performance characteristics of the Large Space Simulator (LSS) at ESTEC, The Netherlands are discussed. The LSS cryogenics system has proven its operational capabilities under simulated heat load conditions and provides sufficient margin for future elevated requirements. The acceptance test proved that nominal operating pressures can be lower than the design parameters, providing increased system safety and reliability. The ease of access for repair and the incorporated redundancy will limit system downtime. Finally, the system design resulted in a low consumption of LN sub 2, which is an important factor in keeping the operational costs at a low level
Life cycle assessment of intensive striped catfish farming in the Mekong Delta for screening hotspots as input to environmental policy and research agenda
Purpose Intensive striped catfish production in the Mekong Delta has, in recent years, raised environmental concerns. We conducted a stakeholder-based screening life cycle assessment (LCA) of the intensive farming system to determine the critical environmental impact and their causative processes in producing striped catfish. Additional to the LCA, we assessed water use and flooding hazards in the Mekong Delta. Materials and methods The goal and scope of the LCA were defined in a stakeholder workshop. It was decided there to include all processes up to the exit-gate of the fish farm in the inventory and to focus life cycle impact assessment on global warming, acidification, eutrophication, human toxicity, and marine (MAET) and freshwater aquatic ecotoxicity (FWET). A survey was used to collect primary inventory data from 28 farms on fish grow-out, and from seven feed mills. Hatching and nursing of striped catfish fingerlings were not included in the assessment due to limited data availability and low estimated impact. Average feed composition for all farms had to be applied due to limitation of budget and data availability. Results and discussion Feed ingredient production, transport and milling dominated most of the impact categories in the LCA except for eutrophication and FWET. Most feed ingredients were produced outside Vietnam, and the impact of transport was important. Because of the screening character of this LCA, generic instead of specific inventory data were used for modelling feed ingredient production. However, the use of generic data is unlikely to have affected the main findings, given the dominance of feed production in all impact categories. Of the feed ingredients, rice bran contributed the most to global warming and acidification, while wheat bran contributed the most to eutrophication. The dominance of both was mainly due to the amounts used. Fishmeal production, transport and energy contributed the most to MAET. The biggest impacts of grow-out farming in Vietnam are on eutrophication and FWET. Water nutrient discharge from grow-out farming was high but negligible compared with the natural nutrient content of the Mekong River. The discharge from all grow-out farms together hardly modified river water quality compared with that before sector expansion. Conclusions Feed production, i.e. ingredient production and transport and milling, remains the main contributor to most impact categories. It contributes indirectly to eutrophication and FWET through the pond effluents. The environmental impact of Pangasius grow-out farming can be reduced by effectively managing sludge and by using feeds with lower feed conversion ratio and lower content of fishery products in the feed. To consider farm variability, a next LCA of aquaculture should enlist closer collaboration from several feed-milling companies and sample farms using their feeds. Future LCAs should also preferably collect specific instead of generic inventory data for feed ingredient production, and include biodiversity and primary production as impact categories
Differential equations for loop integrals in Baikov representation
We present a proof that differential equations for Feynman loop integrals can
always be derived in Baikov representation without involving dimension-shift
identities. We moreover show that in a large class of two- and three-loop
diagrams it is possible to avoid squared propagators in the intermediate steps
of setting up the differential equations.Comment: 11 pages, two-column format, 4 figures. Minor changes; journal
versio
Optimal softening for force calculations in collisionless N-body simulations
In N-body simulations the force calculated between particles representing a
given mass distribution is usually softened, to diminish the effect of
graininess. In this paper we study the effect of such a smoothing, with the aim
of finding an optimal value of the softening parameter. As already shown by
Merritt (1996), for too small a softening the estimates of the forces will be
too noisy, while for too large a softening the force estimates are
systematically misrepresented. In between there is an optimal softening, for
which the forces in the configuration approach best the true forces. The value
of this optimal softening depends both on the mass distribution and on the
number of particles used to represent it. For higher number of particles the
optimal softening is smaller. More concentrated mass distributions necessitate
smaller softening, but the softened forces are never as good an approximation
of the true forces as for not centrally concentrated configurations. We give
good estimates of the optimal softening for homogeneous spheres, Plummer
spheres, and Dehnen spheres. We also give a rough estimate of this quantity for
other mass distributions, based on the harmonic mean distance to the th
neighbour ( = 1, .., 12), the mean being taken over all particles in the
configuration. Comparing homogeneous Ferrers ellipsoids of different shapes we
show that the axial ratios do not influence the value of the optimal softening.
Finally we compare two different types of softening, a spline softening
(Hernquist & Katz 1989) and a generalisation of the standard Plummer softening
to higher values of the exponent. We find that the spline softening fares
roughly as well as the higher powers of the power-law softening and both give a
better representation of the forces than the standard Plummer softening.Comment: 16 pages Latex, 19 figures, accepted for publication in MNRAS,
corrected typos, minor changes mainly in sec.
Platformed professionalization:Labour, assets, and earning a livelihood through Airbnb
This paper analyses processes of professionalization on Airbnb in Berlin, exploring who is able to take part successfully in urban value creation processes facilitated by short-term rental platforms. In doing so, it intervenes in debates on platform urbanism that focus on the role of digital platforms in reconfiguring urban governance and livelihoods. Combining a political economic approach and affordance theory, I conceptualize professionalization as a particular platform logic that benefits Airbnb and hosts who are able to take part, while reinforcing existing inequalities. Drawing on eight months of fieldwork in Berlin, I show how these professionalization practices differentially affect the strategies and practices of hosts, offering benefits to some while worsening the position of others who are unable or unwilling to professionalize. As such, professionalization processes produce inequalities and power asymmetries both on and off the platform, between hosts as well as between the platform owner, platform users, and non-platform users. In a context where a growing number of city-dwellers rely on platforms to generate their livelihoods, such power shifts resulting from programmatic platform dynamics have a significant impact on who is able to benefit from platformization and thrive in a platform society.<br/
Development of B cells in scid mice with immunoglobulin transgenes: Implications for the Control of V(D)J recombination
AbstractThe inability of scid pro-B cells to progress to the pre-B and B cell stages is believed to be caused by a defective recombinase activity that fails to resolve chromosomal breaks resulting from attempted V(D)J recombination. In support of this model, we report that certain immunoglobulin transgenes, specifically those which strongly Inhibit endogenous VH-to-DJH and Vκ-to-Jκ rearrangement in wild-type mice, allow ccid pro-B cells to progress to the pre-B and B cell stages. This rescue of scid B cell differentiation is associated with a dramatic reduction in expression of the recombination activation genes, RAG1 and RAG2, and with reduced transcription of the K locus
The Global Entrepreneurship Monitor United Kingdom 2008 Executive Report
This report compares Global Entrepreneurship Monitor (GEM) measures of entrepreneurial attitudes, activity and aspiration in the UK with participating G7 countries and the large industrialized or industrializing countries of Brazil, Russia, India and China ('BRIC'). It also summarizes entrepreneurial attitudes, activity and aspiration within Government Official Regions of the UK and, for the first time, demonstrates the pattern of entrepreneurial activity at the sub-regional (NUTS2) level
Statistics of the structure components in S0s: implications for bar induced secular evolution
The fractions and dimension of bars, rings and lenses are studied in the
Near-IR S0 galaxy Survey (NIRS0S). We find evidence that multiple lenses in
some barred S0s are related to bar resonances in a similar manner as the inner
and outer rings, for which the outer/inner length ratio 2. Inner lenses in the
non-barred galaxies normalized to galaxy diameter are clearly smaller than
those in the barred systems. Interestingly, these small lenses in the
non-barred galaxies have similar sizes as barlenses (lens-like structures
embedded in a bar), and therefore might actually be barlenses in former barred
galaxies, in which the outer, more elongated bar component, has been destroyed.
We also find that fully developed inner lenses are on average a factor 1.3
larger than bars, whereas inner rings have similar sizes as bars. The fraction
of inner lenses is found to be constant in all family classes (A, AB, B).
Nuclear bars appear most frequently among the weakly barred (AB) galaxies,
which is consistent with the theoretical models by Maciejewski & Athanassoula
(2008). Similar sized bars as the nuclear bars were detected in seven
'non-barred' S0s. Galaxy luminosity does not uniquely define the sizes of bars
or bar-related structures, neither is there any upper limit in galaxy
luminosity for bar formation. Although all the family classes cover the same
range of galaxy luminosity, the non-barred (A) galaxies are on average 0.6 mag
brighter than the strongly barred (B) systems. Overall, our results are
consistent with the idea that bars play an important role in the formation of
the structure components of galaxies. The fact that multiple lenses are common
in S0s, and that at least the inner lenses can have very old stellar
populations, implies that the last destructive merger, or major gas accretion
event, must have taken place at a fairly high redshift.Comment: 36 pages (include 13 figures, 11 tables). Accepted to MNRAS 2013 Jan
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