11,569 research outputs found
Ionizing radiation fluctuations and large-scale structure in the Lyman-alpha forest
We investigate the large-scale inhomogeneities of the hydrogen ionizing
radiation field in the Universe at redshift z=3. Using a raytracing algorithm,
we simulate a model in which quasars are the dominant sources of radiation. We
make use of large scale N-body simulations of a LambdaCDM universe, and include
such effects as finite quasar lifetimes and output on the lightcone, which
affects the shape of quasar light echoes. We create Lya forest spectra that
would be generated in the presence of such a fluctuating radiation field,
finding that the power spectrum of the Lya forest can be suppressed by as much
as 15 % for modes with k=0.05-1 Mpc/h. This relatively small effect may have
consequences for high precision measurements of the Lya power spectrum on
larger scales than have yet been published. We also investigate another
radiation field probe, the cross-correlation of quasar positions and the Lya
forest. For both quasar lifetimes which we simulate (10^7 yr and 10^8 yr), we
expect to see a strong decrease in the Lya absorption close to other quasars
(the ``foreground'' proximity effect). We then use data from the Sloan Digital
Sky Survey First Data Release to make an observational determination of this
statistic. We find no sign of our predicted lack of absorption, but instead
increased absorption close to quasars. If the bursts of radiation from quasars
last on average < 10^6 yr, then we would not expect to be able to see the
foreground effect. However, the strength of the absorption itself seems to be
indicative of rare objects, and hence much longer total times of emission per
quasar. Variability of quasars in bursts with timescales > 10^4yr and < 10^6 yr
could reconcile these two facts.Comment: Submitted to ApJ, 21 pages, 17 postscript figures, emulateapj.st
On the importance of local sources of radiation for quasar absorption line systems
A generic assumption of ionization models of quasar absorption systems is
that radiation from local sources is negligible compared with the cosmological
background. We test this assumption and find that it is unlikely to hold for
absorbers as rare as H I Lyman limit systems. Assuming that the absorption
systems are gas clouds centered on sources of radiation, we derive analytic
estimates for the cross-section weighted moments of the flux seen by the
absorbers, of the impact parameter, and of the luminosity of the central
source. In addition, we compute the corresponding medians numerically. For the
one class of absorbers for which the flux has been measured: damped Ly-alpha
systems at z~3, our prediction is in excellent agreement with the observations
if we assume that the absorption arises in clouds centered on Lyman-break
galaxies. Finally, we show that if Lyman-break galaxies dominate the UV
background at redshift 3, then consistency between observations of the UV
background, the UV luminosity density from galaxies, and the number density of
Lyman limit systems requires escape fractions of order 10 percent.Comment: Accepted for publication in the Astrophysical Journal, 11 pages, 1
figure. Version 2: Added alternative method. Decreased fiducial escape
fraction to guarantee consistency between observed luminosity density, mean
free path, and UV background. This increased the column density above which
local radiation is importan
Su(3) Algebraic Structure of the Cuprate Superconductors Model based on the Analogy with Atomic Nuclei
A cuprate superconductor model based on the analogy with atomic nuclei was
shown by Iachello to have an structure. The mean-field approximation
Hamiltonian can be written as a linear function of the generators of
algebra. Using algebraic method, we derive the eigenvalues of the reduced
Hamiltonian beyond the subalgebras and of
algebra. In particular, by considering the coherence between s- and d-wave
pairs as perturbation, the effects of coherent term upon the energy spectrum
are investigated
Induced Growth of Asymmetric Nanocantilever Arrays on Polar Surfaces
©2003 The American Physical Society. The electronic version of this article is the complete one and can be found online at: http://link.aps.org/doi/10.1103/PhysRevLett.91.185502DOI: 10.1103/PhysRevLett.91.185502We report that the Zn-terminated ZnO (0001) polar surface is chemically active and the oxygenterminated (0001) polar surface is inert in the growth of nanocantilever arrays. Longer and wider "comblike" nanocantilever arrays are grown from the (0001)-Zn surface, which is suggested to be a self-catalyzed process due to the enrichment of Zn at the growth front. The chemically inactive
(0001)-O surface typically does not initiate any growth, but controlling experimental conditions could lead to the growth of shorter and narrower nanocantilevers from the intersections between (0001)-O with (0110) surfaces
Production mechanisms and single-spin asymmetry for kaons in high energy hadron-hadron collisions
Direct consequences on kaon production of the picture proposed in a recent
Letter and subsequent publications are discussed. Further evidence supporting
the proposed picture is obtained. Comparison with the data for the inclusive
cross sections in unpolarized reactions is made. Quantitative results for the
left-right asymmetry in single-spin processes are presented.Comment: 10 pages, 2 Postscript figure
On the Search for Quasar Light Echoes
The UV radiation from a quasar leaves a characteristic pattern in the
distribution of ionized hydrogen throughout the surrounding space. This pattern
or light echo propagates through the intergalactic medium at the speed of
light, and can be observed by its imprint on the Ly-alpha forest spectra of
background sources. As the echo persists after the quasar has switched off, it
offers the possibility of searching for dead quasars, and constraining their
luminosities and lifetimes. We outline a technique to search for and
characterize these light echoes. To test the method, we create artificial
Ly-alpha forest spectra from cosmological simulations at z=3, apply light
echoes and search for them. We show how the simulations can also be used to
quantify the significance level of any detection. We find that light echoes
from the brightest quasars could be found in observational data. With
absorption line spectra of 100 redshift z~3-3.5 quasars or galaxies in a 1
square degree area, we expect that ~10 echoes from quasars with B band
luminosities L_B=3x10^45 ergs/s exist that could be found at 95% confidence,
assuming a quasar lifetime of ~10^7 yr. Even a null result from such a search
would have interesting implications for our understanding of quasar
luminosities and lifetimes.Comment: 9 pages, 7 figures, ApJ in pres
Correlation effects in the ground state charge density of Mott-insulating NiO: a comparison of ab-initio calculations and high-energy electron diffraction measurements
Accurate high-energy electron diffraction measurements of structure factors
of NiO have been carried out to investigate how strong correlations in the Ni
3d shell affect electron charge density in the interior area of nickel ions and
whether the new ab-initio approaches to the electronic structure of strongly
correlated metal oxides are in accord with experimental observations. The
generalized gradient approximation (GGA) and the local spin density
approximation corrected by the Hubbard U term (LSDA+U) are found to provide the
closest match to experimental measurements. The comparison of calculated and
observed electron charge densities shows that correlations in the Ni 3d shell
suppress covalent bonding between the oxygen and nickel sublattices.Comment: 6 pages, LaTeX and 5 figures in the postscript forma
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