13,699 research outputs found
Silicon consumption and population density changes of dominant planktonic diatoms in Lake Constance
(1) Interactions between silicon concentration and diatom growth have been studied in Lake Constance with data on number and volume of cells per unit volume of water and concentration of dissolved reactive silicon. (2) Concentration of dissolved reactive silicon was found to decrease markedly during the spring and summer growth period of diatoms in 1979 and 1980. (3) Amounts of silicon taken up per cell and per unit cell volume have been calculated from the changes in silicon concentration and the increases in population density for several diatom species. The silicon to cell quotient (mg per million cells) varied over a wide range from 0.005 for Stephanodiscus hantzschii to 0.72 for Synedra acus, while that of silicon per unit cell volume (mg mm-3) varied over a narrower range from 0. 10 for Stephanodiscus hantzschii and Fragilaria crotonesis to 0.36 for Synedra acus. (4) A four-compartment model, that allows estimates of the rates of growth and of loss of diatom populations to be calculated, is outlined. (5) Summer succession from Asterionellaformosa to either Fragilaria crotonensis (in 1979) or Stephanodiscus binderanus (in 1980) is explained by interspecific differences in the effects of silicon concentration on growth and death. For Asterionellaformosa, silicon depletion did not stop cell-division but led to the death of most of the population. For Fragilaria crotonensis and Stephanodiscus binderanus cell division stopped but cells did not die and population growth continued after the concentration of dissolved silicon had increased agai
Hadron masses from dynamical, non-perturbatively O(a) improved Wilson fermions
We present results on light hadron masses from simulations of full QCD and
report on experiences in running such simulations on a Hitachi SR8000-F1
supercomputer.Comment: Lattice 2000 (Spectrum), 4 pages, 6 eps figure
Discovery of large scale shock fronts correlated with the radio halo and radio relic in the A2163 galaxy cluster
Imprints of galaxy cluster formation processes are visible in the
intracluster medium and can arise in shock fronts, which are detectable via
discontinuities in e.g. the gas temperature and density profiles. In this
study, we investigate the X-ray properties of the intracluster gas and the
radio morphology of the extraordinary cluster A2163. This cluster shows an
irregular morphology in various wavelengths and has one of the most luminous
and extended known radio halos. Additionally, it is one of the hottest clusters
known. We analyze two Suzaku observations of A2163, one in the north-east (NE)
and one in the south-west (SW) direction, and use archival XMM-Newton data to
remove point sources in the field of view. To compare our findings in the X-ray
regime with the radio emission, we obtain radio images of the cluster from an
archival VLA observation at 20cm. We identify three shock fronts in A2163 in
our spectral X-ray study. A clear shock front lies in the NE direction at a
distance of 1.4Mpc from the center, with a Mach number of
, estimated from the temperature discontinuity. This shock
coincides with the position of a known radio relic. We identify two additional
shocks in the SW direction, one with at a distance of
0.7Mpc, which is likely related to a cool core remnant, and a strong shock with
at a distance of 1.3Mpc, which also closely matches the
radio contours. The complex structure of A2163 as well as the different Mach
numbers and shock velocities suggest a merging scenario with two unequal
merging constituents, where two shock fronts emerged in an early stage of the
merger and traveled outwards while an additional shock front developed in front
of the merging cluster cores.Comment: 9 pages, 7 figures, accepted by A&
The Density Profile of Cluster-scale Dark Matter Halos
We measure the average gravitational shear profile of 6 massive clusters
(M_vir ~ 10^15 M_sun) at z=0.3 out to a radius ~2h^-1 Mpc. The measurements are
fitted to a generalized NFW-like halo model \rho(r) with an arbitrary r -> 0
slope \alpha. The data are well fitted by such a model with a central cusp with
\alpha ~ 0.9 - 1.6 (68% confidence interval). For the standard-NFW case \alpha
= 1.0, we find a concentration parameter c_vir that is consistent with recent
predictions from high-resolution CDM N-body simulations. Our data are also well
fitted by an isothermal sphere model with a softened core. For this model, our
1\sigma upper limit for the core radius corresponds to a limit \sigma_star \leq
0.1 cm^2 g^-1 on the elastic collision cross-section in a self-interacting dark
matter model.Comment: 4 pages, 3 figures; version accepted for publication by ApJ Letters.
Three figures omitted to allow space for new fig. 3 and expanded results and
discussion sections, including NSIS model fi
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