30,954 research outputs found

    Two Aspects of the Mott-Hubbard Transition in Cr-doped V_2O_3

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    The combination of bandstructure theory in the local density approximation with dynamical mean field theory was recently successfully applied to V2_2O3_3 -- a material which undergoes the f amous Mott-Hubbard metal-insulator transition upon Cr doping. The aim of this sh ort paper is to emphasize two aspects of our recent results: (i) the filling of the Mott-Hubbard gap with increasing temperature, and (ii) the peculiarities of the Mott-Hubbard transition in this system which is not characterized by a diver gence of the effective mass for the a1ga_{1g}-orbital.Comment: 2 pages, 3 figures, SCES'04 conference proceeding

    The Small Scale Velocity Dispersion of Galaxies: A Comparison of Cosmological Simulations

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    The velocity dispersion of galaxies on small scales (r∼1h−1r\sim1h^{-1} Mpc), σ12(r)\sigma_{12}(r), can be estimated from the anisotropy of the galaxy-galaxy correlation function in redshift space. We apply this technique to ``mock-catalogs'' extracted from N-body simulations of several different variants of Cold Dark Matter dominated cosmological models to obtain results which may be consistently compared to similar results from observations. We find a large variation in the value of σ12(1h−1Mpc)\sigma_{12}(1 h^{-1} Mpc) in different regions of the same simulation. We conclude that this statistic should not be considered to conclusively rule out any of the cosmological models we have studied. We attempt to make the statistic more robust by removing clusters from the simulations using an automated cluster-removing routine, but this appears to reduce the discriminatory power of the statistic. However, studying σ12\sigma_{12} as clusters with different internal velocity dispersions are removed leads to interesting information about the amount of power on cluster and subcluster scales. We also compute the pairwise velocity dispersion directly and compare this to the values obtained using the Davis-Peebles method, and find that the agreement is fairly good. We evaluate the models used for the mean streaming velocity and the pairwise peculiar velocity distribution in the original Davis-Peebles method by comparing the models with the results from the simulations.Comment: 20 pages, uuencoded (Latex file + 8 Postscript figures), uses AAS macro

    Independent Orbiter Assessment (IOA): Analysis of the life support and airlock support subsystems

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    The results of the Independent Orbiter Assessment (IOA) of the Failure Modes and Effects Analysis (FMEA) and Critical Items List (CIL) are presented. The IOA approach features a top-down analysis of the hardware to determine failure modes, criticality, and potential critical items. To preserve independence, this analysis was accomplished without reliance upon the results contained within the NASA FMEA/CIL documentation. This report documents the independent analysis results corresponding to the Orbiter Life Support System (LSS) and Airlock Support System (ALSS). Each level of hardware was evaluated and analyzed for possible failure modes and effects. Criticality was assigned based upon the severity of the effect for each failure mode. The LSS provides for the management of the supply water, collection of metabolic waste, management of waste water, smoke detection, and fire suppression. The ALSS provides water, oxygen, and electricity to support an extravehicular activity in the airlock

    On the Distribution of Haloes, Galaxies and Mass

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    The stochasticity in the distribution of dark haloes in the cosmic density field is reflected in the distribution function PV(Nh∣δm)P_V(N_h|\delta_m) which gives the probability of finding NhN_h haloes in a volume VV with mass density contrast δm\delta_m. We study the properties of this function using high-resolution NN-body simulations, and find that PV(Nn∣δm)P_V(N_n|\delta_m) is significantly non-Poisson. The ratio between the variance and the mean goes from ∼1\sim 1 (Poisson) at 1+δm≪11+\delta_m\ll 1 to <1<1 (sub-Poisson) at 1+δm∼11+\delta_m\sim 1 to >1>1 (super-Poisson) at 1+δm≫11+\delta_m\gg 1. The mean bias relation is found to be well described by halo bias models based on the Press-Schechter formalism. The sub-Poisson variance can be explained as a result of halo-exclusion while the super-Poisson variance at high δm\delta_m may be explained as a result of halo clustering. A simple phenomenological model is proposed to describe the behavior of the variance as a function of δm\delta_m. Galaxy distribution in the cosmic density field predicted by semi-analytic models of galaxy formation shows similar stochastic behavior. We discuss the implications of the stochasticity in halo bias to the modelling of higher-order moments of dark haloes and of galaxies.Comment: 10 pages, 6 figures, Latex using MN2e style. Minor changes. Accepted for publication in MNRA

    Lagrangian bias in the local bias model

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    It is often assumed that the halo-patch fluctuation field can be written as a Taylor series in the initial Lagrangian dark matter density fluctuation field. We show that if this Lagrangian bias is local, and the initial conditions are Gaussian, then the two-point cross-correlation between halos and mass should be linearly proportional to the mass-mass auto-correlation function. This statement is exact and valid on all scales; there are no higher order contributions, e.g., from terms proportional to products or convolutions of two-point functions, which one might have thought would appear upon truncating the Taylor series of the halo bias function. In addition, the auto-correlation function of locally biased tracers can be written as a Taylor series in the auto-correlation function of the mass; there are no terms involving, e.g., derivatives or convolutions. Moreover, although the leading order coefficient, the linear bias factor of the auto-correlation function is just the square of that for the cross-correlation, it is the same as that obtained from expanding the mean number of halos as a function of the local density only in the large-scale limit. In principle, these relations allow simple tests of whether or not halo bias is indeed local in Lagrangian space. We discuss why things are more complicated in practice. We also discuss our results in light of recent work on the renormalizability of halo bias, demonstrating that it is better to renormalize than not. We use the Lognormal model to illustrate many of our findings.Comment: 14 pages, published on JCA
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