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31.
There is strong evidence that the mass of the Universe is dominated by dark matter, which exerts gravitational attraction but whose exact nature is unknown. In particular, all galaxies are believed to be embedded in massive haloes of dark matter. This view has recently been challenged by the observation of surprisingly low random stellar velocities in the outskirts of ordinary elliptical galaxies, which has been interpreted as indicating a lack of dark matter. Here we show that the low velocities are in fact compatible with galaxy formation in dark-matter haloes. Using numerical simulations of disk-galaxy mergers, we find that the stellar orbits in the outer regions of the resulting ellipticals are very elongated. These stars were torn by tidal forces from their original galaxies during the first close passage and put on outgoing trajectories. The elongated orbits, combined with the steeply falling density profile of the observed tracers, explain the observed low velocities even in the presence of large amounts of dark matter. Projection effects when viewing a triaxial elliptical can lead to even lower observed velocities along certain lines of sight. 相似文献
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Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model 总被引:111,自引:0,他引:111
The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate. About half of the current emissions are being absorbed by the ocean and by land ecosystems, but this absorption is sensitive to climate as well as to atmospheric carbon dioxide concentrations, creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change. Here we present results from a fully coupled, three-dimensional carbon-climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr(-1) is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models, resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback. 相似文献
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Scrapie in Britain during the BSE years 总被引:2,自引:0,他引:2
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The mechanisms by which the adaptive immune system of animals discriminate between foreign and self components are not known. It is generally believed that antibodies are produced only in response to foreign agents. However, against this axiom there is mounting evidence that normal animals produce autoantibodies against a variety of self components. In two murine model systems, a high proportion of IgM-producing cells secrete autoantibodies. In one model, the autoantibodies are specific for isologous mouse IgG(Fc) and in the other, the autoantibodies are specific for antigens revealed on the surface of mouse erythrocytes (RBC) by proteolytic enzymes. The function of these two autoimmune responses is a mystery. Here we show that the two responses are related. The lysis of mouse RBC, modified with bromelain (brom) by IgM autoantibodies was completely inhibited by isologous IgG(Fc), and antibodies against mouse IgG(Fc) formed precipitin bands with solubilized membranes from mouse RBC. We conclude that mouse RBC membranes and IgG(Fc) have at least one antigenic determinant in common and that this determinant is the target for autoimmune responses in normal mice. 相似文献
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