15,916 research outputs found
The Low Column Density Lyman-alpha Forest
We develop an analytical method based on the lognormal approximation to
compute the column density distribution of the Lyman-alpha forest in the low
column density limit. We compute the column density distributions for six
different cosmological models and found that the standard, COBE-normalized CDM
model cannot fit the observations of the Lyman-alpha forest at z=3. The
amplitude of the fluctuations in that model has to be lowered by a factor of
almost 3 to match observations. However, the currently viable cosmological
models like the lightly tilted COBE-normalized CDM+Lambda model, the CHDM model
with 20% neutrinos, and the low-amplitude Standard CDM model are all in
agreement with observations, to within the accuracy of our approximation, for
the value of the cosmological baryon density at or higher than the old Standard
Bing Bang Nucleosynthesis value of 0.0125 for the currently favored value of
the ionizing radiation intensity. With the low value for the baryon density
inferred by Hogan & Rugers (1996), the models can only marginally match
observations.Comment: three postscript figures included, submitted to ApJ
Large-Scale Structure Shocks at Low and High Redshifts
Cosmological simulations show that, at the present time, a substantial
fraction of the gas in the intergalactic medium (IGM) has been shock-heated to
T>10^5 K. Here we develop an analytic model to describe the fraction of
shocked, moderately overdense gas in the IGM. The model is an extension of the
Press & Schechter (1974) description for the mass function of halos: we assume
that large-scale structure shocks occur at a fixed overdensity during nonlinear
collapse. This in turn allows us to compute the fraction of gas at a given
redshift that has been shock-heated to a specified temperature. We show that,
if strong shocks occur at turnaround, our model provides a reasonable
description of the temperature distribution seen in cosmological simulations at
z~0, although it does overestimate the importance of weak shocks. We then apply
our model to shocks at high redshifts. We show that, before reionization, the
thermal energy of the IGM is dominated by large-scale structure shocks (rather
than virialized objects). These shocks can have a variety of effects, including
stripping ~10% of the gas from dark matter minihalos, accelerating cosmic rays,
and creating a diffuse radiation background from inverse Compton and cooling
radiation. This radiation background develops before the first stars form and
could have measurable effects on molecular hydrogen formation and the spin
temperature of the 21 cm transition of neutral hydrogen. Finally, we show that
shock-heating will also be directly detectable by redshifted 21 cm measurements
of the neutral IGM in the young universe.Comment: 12 pages, 8 figures, submitted to Ap
Spectral properties of the 2D Holstein t-J model
Employing the Lanczos algorithm in combination with a kernel polynomial
moment expansion (KPM) and the maximum entropy method (MEM), we show a way of
calculating charge and spin excitations in the Holstein t-J model, including
the full quantum nature of phonons. To analyze polaron band formation we
evaluate the hole spectral function for a wide range of electron-phonon
coupling strengths. For the first time, we present results for the optical
conductivity of the 2D Holstein t-J model.Comment: 2 pages, Latex. Submitted to Physica C, Proc. Int. Conf. on M2HTSC
Polaronic effects in strongly coupled electron-phonon systems: Exact diagonalization results for the 2D Holstein t-J model
Ground-state and dynamical properties of the 2D Holstein t-J model are
examined by means of direct Lanczos diagonalization, using a truncation method
of the phononic Hilbert space. The single-hole spectral function shows the
formation of a narrow hole-polaron band as the electron-phonon coupling
increases, where the polaronic band collapse is favoured by strong Coulomb
correlations. In the two-hole sector, the hole-hole correlations unambiguously
indicate the existence of inter-site bipolaronic states. At quarter-filling, a
polaronic superlattice is formed in the adiabatic strong-coupling regime.Comment: 3 pages, LaTeX, 6 Postscript figures, Proc. Int. Conf. on Strongly
Correlated Electron Systems, Zuerich, August 1996, accepted for publication
in Physica
Ultraviolet Line Emission from Metals in the Low-Redshift Intergalactic Medium
We use a high-resolution cosmological simulation that includes hydrodynamics,
multiphase star formation, and galactic winds to predict the distribution of
metal line emission at z~0 from the intergalactic medium (IGM). We focus on two
ultraviolet doublet transitions, OVI 1032,1038 and CIV 1548,1551. Emission from
filaments with moderate overdensities is orders of magnitude smaller than the
background, but isolated emission from enriched, dense regions with
T~10^5-10^5.5 K and characteristic sizes of 50-100 kpc can be detected above
the background. We show that the emission from these regions is substantially
greater when we use the metallicities predicted by the simulation (which
includes enrichment through galactic winds) than when we assume a uniform IGM
metallicity. Luminous regions correspond to volumes that have recently been
influenced by galactic winds. We also show that the line emission is clustered
on scales ~1 h^-1 Mpc. We argue that although these transitions are not
effective tracers of the warm-hot intergalactic medium, they do provide a route
to study the chemical enrichment of the IGM and the physics of galactic winds.Comment: replaced by version to appear in ApJ (conclusions unchanged, one new
figure), 16 pages (emulateapj), 11 figures, version with higher resolution
figures available at
http://www.tapir.caltech.edu/~sfurlane/metals/coverpage.htm
Quantum measurement of coherence in coupled quantum dots
We describe the conditional and unconditional dynamics of two coupled quantum
dots when one dot is subjected to a measurement of its occupation number using
a single electron transistor (SET). The measurement is made when the bare
tunneling rate though the SET is changed by the occupation number of one of the
dots. We show that there is a difference between the time scale for the
measurement-induced decoherence between the localized states of the dots and
the time scale on which the system becomes localized due to the measurement. A
comparison between theory and current experiments is made.Comment: 12 pages, 7 figure
Intermittent Features of the QSO Ly Transmitted Flux: Results from Hydrodynamic Cosmological Simulations
It has been recently found that the local fluctuations of the QSO's
Ly absorption spectrum transmitted flux show spiky structures. This
implies that the mass fields of the intergalactic medium (IGM) is intermittent.
This feature cannot be explained by the clustering evolution of cosmic mass
field in the linear regimes and is also difficult to incorporate into the
hierarchical clustering scenario. We calculate the structure functions and
intermittent exponent of the IGM and HI for full hydrodynamical simulation
samples. The result shows the intermittent features of the Ly
transmitted flux fluctuations as well as the mass field of the IGM. We find
that within the error bars of current data, all the intermittent behavior of
the simulation samples are consistent with the observation. This result is
different from our earlier result (Pando et al 2002), which shows that the
intermittent behavior of samples generated by pseudo-hydro simulation cannot be
fitted with observed data. One difference between the pseudo-hydro and full
hydro simulations is in treating the dynamical relation between the IGM (or HI)
and dark matter fields. The former assumes that the IGM density distribution
traces the underlying dark matter point-by-point on scales larger than the
Jeans length in either the linear or nonlinear regimes. However, hydrodynamic
studies have found that a statistical discrepancy between the IGM field and
underlying dark matter in nonlinear regime is possible. We find that the
point-by-point correlation between the IGM density perturbations and dark
matter become weaker on comoving scales less than 2 h Mpc (in LCDM
model), which is larger than the IGM Jeans length.Comment: AAS Latex file, 38 pages,17 figures included, accepted for
publication in Ap
The Evolution of Optical Depth in the Ly-alpha Forest: Evidence Against Reionization at z~6
We examine the evolution of the IGM Ly-alpha optical depth distribution using
the transmitted flux probability distribution function (PDF) in a sample of 63
QSOs spanning absorption redshifts 1.7 < z < 5.8. The data are compared to two
theoretical optical depth distributions: a model distribution based on the
density distribution of Miralda-Escude et al. (2000) (MHR00), and a lognormal
distribution. We assume a uniform UV background and an isothermal IGM for the
MHR00 model, as has been done in previous works. Under these assumptions, the
MHR00 model produces poor fits to the observed flux PDFs at redshifts where the
optical depth distribution is well sampled, unless large continuum corrections
are applied. However, the lognormal optical depth distribution fits the data at
all redshifts with only minor continuum adjustments. We use a simple
parametrization for the evolution of the lognormal parameters to calculate the
expected mean transmitted flux at z > 5.4. The lognormal optical depth
distribution predicts the observed Ly-alpha and Ly-beta effective optical
depths at z > 5.7 while simultaneously fitting the mean transmitted flux down
to z = 1.6. If the evolution of the lognormal distribution at z < 5 reflects a
slowly-evolving density field, temperature, and UV background, then no sudden
change in the IGM at z ~ 6 due to late reionization appears necessary. We have
used the lognormal optical depth distribution without any assumption about the
underlying density field. If the MHR00 density distribution is correct, then a
non-uniform UV background and/or IGM temperature may be required to produce the
correct flux PDF. We find that an inverse temperature-density relation greatly
improves the PDF fits, but with a large scatter in the equation of state index.
[Abridged]Comment: 45 pages, 16 figures, submitted to Ap
Robustness and Enhancement of Neural Synchronization by Activity-Dependent Coupling
We study the synchronization of two model neurons coupled through a synapse
having an activity-dependent strength. Our synapse follows the rules of
Spike-Timing Dependent Plasticity (STDP). We show that this plasticity of the
coupling between neurons produces enlarged frequency locking zones and results
in synchronization that is more rapid and much more robust against noise than
classical synchronization arising from connections with constant strength. We
also present a simple discrete map model that demonstrates the generality of
the phenomenon.Comment: 4 pages, accepted for publication in PR
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