333 research outputs found
Numerical simulation of the clustering of constant-volume balloons in the global domain
Numerical simulation of motion and clustering of balloons originally randomly spaced in upper and lower tropospher
Numerical simulation of the 1970 to 1971 Eole experiment
Numerical simulation of global atmospheric circulation for predicting performance of balloons in Eole experimen
Ionization near-zones associated with quasars at z ~ 6
We analyze the size evolution of HII regions around 27 quasars between z=5.7
to 6.4 ('quasar near-zones' or NZ). We include more sources than previous
studies, and we use more accurate redshifts for the host galaxies, with 8 CO
molecular line redshifts and 9 MgII redshifts. We confirm the trend for an
increase in NZ size with decreasing redshift, with the luminosity normalized
proper size evolving as: R_{NZ,corrected} = (7.4 \pm 0.3) - (8.0 \pm 1.1)
\times (z-6) Mpc. While derivation of the absolute neutral fraction remains
difficult with this technique, the evolution of the NZ sizes suggests a
decrease in the neutral fraction of intergalactic hydrogen by a factor ~ 9.4
from z=6.4 to 5.7, in its simplest interpretation. Alternatively, recent
numerical simulations suggest that this rapid increase in near-zone size from
z=6.4 to 5.7 is due to the rapid increase in the background photo-ionization
rate at the end of the percolation or overlap phase, when the average mean free
path of ionizing photons increases dramatically. In either case, the results
are consistent with the idea that z ~ 6 to 7 corresponds to the tail end of
cosmic reionization. The scatter in the normalized NZ sizes is larger than
expected simply from measurement errors, and likely reflects intrinsic
differences in the quasars or their environments. We find that the near-zone
sizes increase with quasar UV luminosity, as expected for photo-ionization
dominated by quasar radiation.Comment: 16 pages, aas format, 4 figures, to appear in the ApJ letter
Evaluating Sensitivities of Economic Factors through Coupled Economics-ALMANAC Model System
Using crop models to simulate crop growth and productivity at a regional scale is a complex process designed to represent the observed impact of individual farmer decision-making on the agricultural landscape. Typically, during agricultural simulation efforts, the planting acreages have largely been based on a set of predetermined, static scenarios. In this study, we developed a system to dynamically enhance the Agricultural Land Management Alternative with Numerical Assessment Criteria (ALMANAC) crop simulation model through a two-way linkage with an economics land-use model. This coupled model approach integrated farmers’ land-use choices based on relative economic returns and produced dynamic land-use probabilities for ALMANAC simulations through a feedback loop. The coupled model approach was intercompared with static crop modeling through a historic acreage approach, and comparable accuracies were found from both modeling efforts for the 2014 growing season. Furthermore, as a proof-of-concept effort, the method was applied to evaluate the impact of two scenarios on crop simulations: major crops (maize, soybean, and wheat) intensification through price increases (e.g., market change) and incentivized grassland conservation (e.g., policy change). The results of this sensitivity study suggest that the coupled system has the capability to integrate economic factors into traditional crop simulation, allowing for insight into the impacts of changes in markets and policies on agricultural landscapes and crop yields
The Redshift Distribution of Distant Supernovae and its Use in Probing Reionization
We model the number of detectable supernovae (SNe) as a function of redshift
at different flux thresholds, making use of the observed properties of local
SNe, such as their lightcurves, fiducial spectra, and peak magnitude
distributions. We obtain a rate of 0.4-2.3 SNe/arcmin^2/yr at z > 5 at the near
infrared (4.5 micron) flux density threshold of 3 nJy (achievable with the
James Webb Space Telescope (JWST) in a 10^5 s integration). In a hypothetical
one-year survey, it should be possible to detect up to several thousand SNe per
unit redshift at z > 6. We discuss the possible application of such a large
sample of distant SNe as a probe of the epoch of reionization. By heating the
intergalactic medium and raising the cosmological Jeans mass, the process of
reionization can suppress star formation in low-mass galaxies. This could have
produced a relatively sharp drop in the SNR around the redshift of
reionization. We quantify the detectability of this feature in future surveys
of distant SNe by varying the redshift and duration of the feature, as well as
its impact on the SFR in low-mass halos, which results in different redshifts,
widths, and sizes of the drop in the expected SFR. We find that the drop can be
detected out to z_re ~ 13, as long as (i) the reionization history contains a
relatively rapid feature which is synchronized over different regions to within
\Delta z < 1 - 3, (ii) the star-formation efficiency in halos that dominate
reionization is ~ 10%, and (iii) reionization significantly suppresses the star
formation in low-mass halos. Depending on the details of (i) - (iii), this
could be achieved with a survey lasting less than two weeks. Detecting this
signature would also help elucidate the feedback mechanism that regulates
reionization.Comment: 11 pages, 9 figures; ApJ accepte
Polyakov Loops versus Hadronic States
The order parameter for the pure Yang-Mills phase transition is the Polyakov
loop which encodes the symmetries of the Z_N center of the SU(N) gauge group.
On the other side the physical degrees of freedom of any asymptotically free
gauge theory are hadronic states. Using the Yang-Mills trace anomaly and the
exact Z_N symmetry we construct a model able to communicate to the hadrons the
information carried by the order parameter.Comment: RevTex4 2-col., 6 pages, 2 figures. Typos fixed and added a paragraph
in the conclusion
PAPER-64 Constraints On Reionization II: The Temperature Of The z=8.4 Intergalactic Medium
We present constraints on both the kinetic temperature of the intergalactic
medium (IGM) at z=8.4, and on models for heating the IGM at high-redshift with
X-ray emission from the first collapsed objects. These constraints are derived
using a semi-analytic method to explore the new measurements of the 21 cm power
spectrum from the Donald C. Backer Precision Array for Probing the Epoch of
Reionization (PAPER), which were presented in a companion paper, Ali et al.
(2015). Twenty-one cm power spectra with amplitudes of hundreds of mK^2 can be
generically produced if the kinetic temperature of the IGM is significantly
below the temperature of the Cosmic Microwave Background (CMB); as such, the
new results from PAPER place lower limits on the IGM temperature at z=8.4.
Allowing for the unknown ionization state of the IGM, our measurements find the
IGM temperature to be above ~5 K for neutral fractions between 10% and 85%,
above ~7 K for neutral fractions between 15% and 80%, or above ~10 K for
neutral fractions between 30% and 70%. We also calculate the heating of the IGM
that would be provided by the observed high redshift galaxy population, and
find that for most models, these galaxies are sufficient to bring the IGM
temperature above our lower limits. However, there are significant ranges of
parameter space that could produce a signal ruled out by the PAPER
measurements; models with a steep drop-off in the star formation rate density
at high redshifts or with relatively low values for the X-ray to star formation
rate efficiency of high redshift galaxies are generally disfavored. The PAPER
measurements are consistent with (but do not constrain) a hydrogen spin
temperature above the CMB temperature, a situation which we find to be
generally predicted if galaxies fainter than the current detection limits of
optical/NIR surveys are included in calculations of X-ray heating.Comment: companion paper to Ali et al. (2015), ApJ 809, 61; matches version
accepted to ApJ; 11 pages, 7 figure
Light from the Cosmic Frontier: Gamma-Ray Bursts
Gamma-Ray Bursts (GRBs) are the most powerful cosmic explosions since the Big
Bang, and thus act as signposts throughout the distant Universe. Over the last
2 decades, these ultra-luminous cosmological explosions have been transformed
from a mere curiosity to essential tools for the study of high-redshift stars
and galaxies, early structure formation and the evolution of chemical elements.
In the future, GRBs will likely provide a powerful probe of the epoch of
reionisation of the Universe, constrain the properties of the first generation
of stars, and play an important role in the revolution of multi-messenger
astronomy by associating neutrinos or gravitational wave (GW) signals with
GRBs. Here, we describe the next steps needed to advance the GRB field, as well
as the potential of GRBs for studying the Early Universe and their role in the
up-coming multi-messenger revolution.Comment: White paper submitted to ESA as a contribution to the deliberations
on the science themes for the L2 and L3 mission opportunitie
Observing the First Stars and Black Holes
The high sensitivity of JWST will open a new window on the end of the
cosmological dark ages. Small stellar clusters, with a stellar mass of several
10^6 M_sun, and low-mass black holes (BHs), with a mass of several 10^5 M_sun
should be directly detectable out to redshift z=10, and individual supernovae
(SNe) and gamma ray burst (GRB) afterglows are bright enough to be visible
beyond this redshift. Dense primordial gas, in the process of collapsing from
large scales to form protogalaxies, may also be possible to image through
diffuse recombination line emission, possibly even before stars or BHs are
formed. In this article, I discuss the key physical processes that are expected
to have determined the sizes of the first star-clusters and black holes, and
the prospect of studying these objects by direct detections with JWST and with
other instruments. The direct light emitted by the very first stellar clusters
and intermediate-mass black holes at z>10 will likely fall below JWST's
detection threshold. However, JWST could reveal a decline at the faint-end of
the high-redshift luminosity function, and thereby shed light on radiative and
other feedback effects that operate at these early epochs. JWST will also have
the sensitivity to detect individual SNe from beyond z=10. In a dedicated
survey lasting for several weeks, thousands of SNe could be detected at z>6,
with a redshift distribution extending to the formation of the very first stars
at z>15. Using these SNe as tracers may be the only method to map out the
earliest stages of the cosmic star-formation history. Finally, we point out
that studying the earliest objects at high redshift will also offer a new
window on the primordial power spectrum, on 100 times smaller scales than
probed by current large-scale structure data.Comment: Invited contribution to "Astrophysics in the Next Decade: JWST and
Concurrent Facilities", Astrophysics & Space Science Library, Eds. H.
Thronson, A. Tielens, M. Stiavelli, Springer: Dordrecht (2008
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