12,813 research outputs found
REGULATORY TARGETS AND REGIMES FOR FOOD SAFETY: A COMPARISON OF NORTH AMERICAN AND EUROPEAN APPROACHES
Food quality, international trade, harmonization, mutual recognition, Food Consumption/Nutrition/Food Safety,
The Lyman-alpha Forest as a Cosmological Tool
We review recent developments in the theory of the Lyman-alpha forest and
their implications for the role of the forest as a test of cosmological models.
Simulations predict a relatively tight correlation between the local Lya
optical depth and the local gas or dark matter density. Statistical properties
of the transmitted flux can constrain the amplitude and shape of the matter
power spectrum at high redshift, test the assumption of Gaussian initial
conditions, and probe the evolution of dark energy by measuring the Hubble
parameter H(z). Simulations predict increased Lya absorption in the vicinity of
galaxies, but observations show a Lya deficit within Delta_r ~ 0.5 Mpc/h
(comoving). We investigate idealized models of "winds" and find that they must
eliminate neutral hydrogen out to comoving radii ~1.5 Mpc/h to marginally
explain the data. Winds of this magnitude suppress the flux power spectrum by
\~0.1 dex but have little effect on the distribution function or threshold
crossing frequency. In light of the stringent demands on winds, we consider the
alternative possibility that extended Lya emission from target galaxies
replaces absorbed flux, but we conclude that this explanation is unlikely.
Taking full advantage of the data coming from large telescopes and from the
Sloan Digital Sky Survey will require more complete understanding of the galaxy
proximity effect, careful attention to continuum determination, and more
accurate numerical predictions, with the goal of reaching 5-10% precision on
key cosmological quantities.Comment: 13 pages, to appear in "The Emergence of Cosmic Structure,"
Proceedings of the 13th Annual Astrophysics Conference in Maryland, eds. S.
Holt and C. Reynolds, AIP Press, 200
The Galaxy Proximity Effect in the Lyman-alpha Forest
Hydrodynamic cosmological simulations predict that the average opacity of the
Ly-alpha forest should increase in the neighborhood of galaxies because
galaxies form in dense environments. Recent observations (Adelberger et al.
2002) confirm this expectation at large scales, but they show a decrease of
absorption at comoving separations Delta_r <~ 1 Mpc/h. We show that this
discrepancy is statistically significant, especially for the innermost data
point at Delta_r <= 0.5 Mpc/h, even though this data point rests on three
galaxy-quasar pairs. Galaxy redshift errors of the expected magnitude are
insufficient to resolve the conflict. Peculiar velocities allow gas at comoving
distances >~ 1 Mpc/h to produce saturated absorption at the galaxy redshift,
putting stringent requirements on any ``feedback'' solution. Local
photoionization is insufficient, even if we allow for recurrent AGN activity
that keeps the neutral hydrogen fraction below its equilibrium value. A simple
``wind'' model that eliminates all neutral hydrogen in spheres around the
observed galaxies can marginally explain the data, but only if the winds extend
to comoving radii ~1.5 Mpc/h.Comment: 4 pages, 1 figure; To appear in proceedings of the 13th Annual
Astrophysics Conference in College Park, Maryland, The Emergence of Cosmic
Structure, eds. S.Holt and C. Reynolds, (AIP
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
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
Sub-mm Galaxies in Cosmological Simulations
We study the predicted sub-mm emission from massive galaxies in a Lambda-CDM
universe, using hydrodynamic cosmological simulations. Assuming that most of
the emission from newly formed stars is absorbed and reradiated in the
rest-frame far-IR, we calculate the number of galaxies that would be detected
in sub-mm surveys conducted with SCUBA. The predicted number counts are
strongly dependent on the assumed dust temperature and emissivity law. With
plausible choices for SED parameters (e.g., T=35 K, beta=1.0), the simulation
predictions reproduce the observed number counts above ~ 1 mJy. The sources
have a broad redshift distribution with median z ~ 2, in reasonable agreement
with observational constraints. However, the predicted count distribution may
be too steep at the faint end, and the fraction of low redshift objects may be
larger than observed.
In this physical model of the sub-mm galaxy population, the objects detected
in existing surveys consist mainly of massive galaxies (several M_*) forming
stars fairly steadily over timescales ~ 10^8-10^9 years, at moderate rates ~100
Msun/yr. The typical descendants of these sub-mm sources are even more massive
galaxies, with old stellar populations, found primarily in dense environments.
While the resolution of our simulations is not sufficient to determine galaxy
morphologies, these properties support the proposed identification of sub-mm
sources with massive ellipticals in the process of formation. The most robust
and distinctive prediction of this model, stemming directly from the long
timescale and correspondingly moderate rate of star formation, is that the
far-IR SEDs of SCUBA sources have a relative high 850 micron luminosity for a
given bolometric luminosity. [Abridged]Comment: Submitted to ApJ. 34 pages including 8 PS figure
Statistical properties of fractures in damaged materials
We introduce a model for the dynamics of mud cracking in the limit of of
extremely thin layers. In this model the growth of fracture proceeds by
selecting the part of the material with the smallest (quenched) breaking
threshold. In addition, weakening affects the area of the sample neighbour to
the crack. Due to the simplicity of the model, it is possible to derive some
analytical results. In particular, we find that the total time to break down
the sample grows with the dimension L of the lattice as L^2 even though the
percolating cluster has a non trivial fractal dimension. Furthermore, we obtain
a formula for the mean weakening with time of the whole sample.Comment: 5 pages, 4 figures, to be published in Europhysics Letter
The Mah Jongg Nightmare
Sheet music contains anti-Asian racist language, stereotypes, and/or imagry.https://digitalcommons.library.umaine.edu/mmb-vp/6871/thumbnail.jp
- …