25,211 research outputs found
The preparation, identification and properties of chlorophyll derivatives
In the investigation of 10-hydroxy chlorophylls a and b novel techniques included modification of chromatography and the use of fully-deuterated compounds isolated from fully-deuterated autotropic algae to determine the molecular structure of the chlorophylls
Development of a magneforming process for the fabrication of thin-wall tungsten cylinders final report
Magneforming process - high energy rate metal forming technique for fabrication of thin wall tungsten cylinder
Intergalactic Helium Absorption in Cold Dark Matter Models
Observations from the HUT and the HST have recently detected HeII absorption
along the lines of sight to two high redshift quasars. We use cosmological
simulations with gas dynamics to investigate HeII absorption in the cold dark
matter (CDM) theory of structure formation. We consider two Omega=1 CDM models
with different normalizations and one Omega_0=0.4 CDM model, all incorporating
the photoionizing UV background spectrum computed by Haardt & Madau (1996). The
simulated gas distribution, combined with the H&M spectral shape, accounts for
the relative observed values of taubar_HI and taubar_HeII, the effective mean
optical depths for HI and HeII absorption. If the background intensity is as
high as H&M predict, then matching the absolute values of taubar_HI and
taubar_HeII requires a baryon abundance larger (by factors between 1.5 and 3
for the various CDM models) than our assumed value of Omega_b h^2=0.0125. The
simulations reproduce the evolution of taubar_heII over the observed redshift
range, 2.2 < z < 3.3, if the HeII photoionization rate remains roughly
constant. HeII absorption in the CDM simulations is produced by a diffuse,
fluctuating, intergalactic medium, which also gives rise to the HI ly-alpha
forest. Much of the HeII opacity arises in underdense regions where the HI
optical depth is very low. We compute statistical properties of the HeII and HI
absorption that can be used to test the CDM models and distinguish them from an
alternative scenario in which the HeII absorption is caused by discrete,
compact clouds. The CDM scenario predicts that a substantial amount of baryonic
material resides in underdense regions at high redshift. HeII absorption is the
only sensitive probe of such extremely diffuse, intergalactic gas, so it can
provide a vital test of this fundamental prediction.Comment: Accepted for publication in ApJ, 36 pages, LaTeX (aaspp4), 12
figures. Changes include addition of more information on statistical
uncertainties and on the adopted UV background. Also available at
http://www-astronomy.mps.ohio-state.edu/~racc
Gravitational energy
Observers at rest in a stationary spacetime flat at infinity can measure
small amounts of rest-mass+internal energies+kinetic energies+pressure energy
in a small volume of fluid attached to a local inertial frame. The sum of these
small amounts is the total "matter energy" for those observers. The total
mass-energy minus the matter energy is the binding gravitational energy.
Misner, Thorne and Wheeler evaluated the gravitational energy of a
spherically symmetric static spacetime. Here we show how to calculate
gravitational energy in any static and stationary spacetime for isolated
sources with a set of observers at rest.
The result of MTW is recovered and we find that electromagnetic and
gravitational 3-covariant energy densities in conformastatic spacetimes are of
opposite signs. Various examples suggest that gravitational energy is negative
in spacetimes with special symmetries or when the energy-momentum tensor
satisfies usual energy conditions.Comment: 12 pages. Accepted for publication in Class. Quantum Gra
Industrial Relations and Productivity in the U.S. Automobile Industry
macroeconomics, Automobile, industrial relations, productivity
Characterization of Lyman Alpha Spectra and Predictions of Structure Formation Models: A Flux Statistics Approach
In gravitational instability models, \lya absorption arises from a continuous
fluctuating medium, so that spectra provide a non-linear one-dimensional
``map'' of the underlying density field. We characterise this continuous
absorption using statistical measures applied to the distribution of absorbed
flux. We describe two simple members of a family of statistics which we apply
to simulated spectra in order to show their sensitivity as probes of
cosmological parameters (H, , the initial power spectrum of
matter fluctuations) and the physical state of the IGM. We make use of SPH
simulation results to test the flux statistics, as well as presenting a
preliminary application to Keck HIRES data.Comment: Contribution to proceedings of the 18th Texas Symposium on
Relativistic Astrophysics (eds A. Olinto, J. Frieman and D. Schramm, World
Scientific),Chicago, December 1996, 3 pages, LaTeX (sprocl), 2 figures. Also
available at http://www-astronomy.mps.ohio-state.edu/~racc
Accurate determination of the Lagrangian bias for the dark matter halos
We use a new method, the cross power spectrum between the linear density
field and the halo number density field, to measure the Lagrangian bias for
dark matter halos. The method has several important advantages over the
conventional correlation function analysis. By applying this method to a set of
high-resolution simulations of 256^3 particles, we have accurately determined
the Lagrangian bias, over 4 magnitudes in halo mass, for four scale-free models
with the index n=-0.5, -1.0, -1.5 and -2.0 and three typical CDM models. Our
result for massive halos with ( is a characteristic non-linear
mass) is in very good agreement with the analytical formula of Mo & White for
the Lagrangian bias, but the analytical formula significantly underestimates
the Lagrangian clustering for the less massive halos $M < M_*. Our simulation
result however can be satisfactorily described, with an accuracy better than
15%, by the fitting formula of Jing for Eulerian bias under the assumption that
the Lagrangian clustering and the Eulerian clustering are related with a linear
mapping. It implies that it is the failure of the Press-Schechter theories for
describing the formation of small halos that leads to the inaccuracy of the Mo
& White formula for the Eulerian bias. The non-linear mapping between the
Lagrangian clustering and the Eulerian clustering, which was speculated as
another possible cause for the inaccuracy of the Mo & White formula, must at
most have a second-order effect. Our result indicates that the halo formation
model adopted by the Press-Schechter theories must be improved.Comment: Minor changes; accepted for publication in ApJ (Letters) ; 11 pages
with 2 figures include
On the mass of a Kerr-anti-de Sitter spacetime in D dimensions
We show how to compute the mass of a Kerr-anti-de Sitter spacetime with
respect to the anti-de Sitter background in any dimension, using a
superpotential which has been derived from standard Noether identities. The
calculation takes no account of the source of the curvature and confirms
results obtained for black holes via the first law of thermodynamics.Comment: minor changes; accepted by CQ
The growth of galaxies in cosmological simulations of structure formation
We use hydrodynamic simulations to examine how the baryonic components of
galaxies are assembled, focusing on the relative importance of mergers and
smooth accretion in the formation of ~L_* systems. In our primary simulation,
which models a (50\hmpc)^3 comoving volume of a Lambda-dominated cold dark
matter universe, the space density of objects at our (64-particle) baryon mass
resolution threshold, M_c=5.4e10 M_sun, corresponds to that of observed
galaxies with L~L_*/4. Galaxies above this threshold gain most of their mass by
accretion rather than by mergers. At the redshift of peak mass growth, z~2,
accretion dominates over merging by about 4:1. The mean accretion rate per
galaxy declines from ~40 M_sun/yr at z=2 to ~10 M_sun/yr at z=0, while the
merging rate peaks later (z~1) and declines more slowly, so by z=0 the ratio is
about 2:1. We cannot distinguish truly smooth accretion from merging with
objects below our mass resolution threshold, but extrapolating our measured
mass spectrum of merging objects, dP/dM ~ M^a with a ~ -1, implies that
sub-resolution mergers would add relatively little mass. The global star
formation history in these simulations tracks the mass accretion rate rather
than the merger rate. At low redshift, destruction of galaxies by mergers is
approximately balanced by the growth of new systems, so the comoving space
density of resolved galaxies stays nearly constant despite significant mass
evolution at the galaxy-by-galaxy level. The predicted merger rate at z<~1
agrees with recent estimates from close pairs in the CFRS and CNOC2 redshift
surveys.Comment: Submitted to ApJ, 35 pp including 15 fig
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