51,499 research outputs found
New Measurements of the Ionizing Ultraviolet Background over 2 < z < 5 and Implications for Hydrogen Reionization
We present new measurements of the intensity of the ionizing ultraviolet
background and the global emissivity of ionizing photons over 2 < z < 5. Our
results are based on a suite of updated measurements of physical properties of
the high-redshift intergalactic medium (IGM), including gas temperatures and
the opacity of the IGM to Ly-alpha and ionizing photons. Consistent with
previous works, we find a relatively flat hydrogen photoionization rate over 2
< z < 5, although our measurements are roughly a factor of two higher than the
2008 values of Faucher-Giguere et al., due primarily to our lower gas
temperatures. The ionizing emissivity we derive is also generally higher than
other recent estimates due to a combination of lower gas temperatures, higher
ionizing opacity, and an accounting of cosmological radiative transfer effects.
We find evidence that the emissivity increases from z~3 to 5, reaching ~5
ionizing photons per atom per gigayear at z=4.75 for realistic galaxy spectra.
We further find that galaxies must dominate the emissivity near 1 Ryd at z > 4,
and possibly at all redshifts z > 2.4. Our results suggest that the
globally-averaged ionizing "efficiency" of star-forming galaxies increases
substantially with redshift over 3.2 < z < 4.75. This trend is consistent with
the conclusion often drawn from reionization models that the ionizing
efficiency of galaxies must be higher during reionization in order for galaxies
to reionize the IGM by z=6. Our emissivity values at z~5 suggest that ionizing
photons may have been a factor of two more abundant during the final stages of
reionization than previously indicated. The evolution of the ionizing
emissivity over 2 < z < 5 suggests, moreover, that the steep decline in the
photoionization rate from z~5 to 6 may indicate a rapid evolution in the mean
free path at z > 5.Comment: 19 pages, 14 figures, MNRAS, in pres
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Presynaptic facilitation at the crayfish neuromuscular junction: Role of calcium-activated potassium conductance
Membrane potential was recorded intracellularly near presynaptic
terminals of the excitor axon of the crayfish opener neuromuscular junction (NMJ),
while transmitter release was recorded postsynaptically. This study focused on the
effects of a presynaptic calcium-activated potassium conductance, gK(Ca), on the
transmitter release evoked by single and paired depolarizing current pulses.
Blocking gK(Ca) by adding tetraethylammonium ion (TEA; 5-20 mM) to a solution
containing tetrodotoxin and aminopyridines caused the relation between presynaptic
potential and transmitter release to steepen and shift to less depolarized
potentials. When two depolarizing current pulses were applied at 20-ms intervals
with gK(Ca) not blocked, the presynaptic voltage change to the second (test) pulse was
inversely related to the amplitude of the first (conditioning) pulse. This effect of the
conditioning prepulse on the response to the test pulse was eliminated by 20 mM
TEA and by solutions containing 0 mM Ca2+/1 mM EGTA, suggesting that the
reduction in the amplitude of the test pulse was due to activation of gK(Ca) by calcium
remaining from the conditioning pulse. In the absence of TEA, facilitation of
transmitter release evoked by a test pulse increased as the conditioning pulse grew
from -40 to -20 mV, but then decreased with further increase in the conditioning
depolarization. A similar nonmonotonic relationship between facilitation and the
amplitude of the conditioning depolarization was reported in previous studies using
extracellular recording, and interpreted as supporting an additional voltagedependent
step in the activation of transmitter release. We suggest that this result
was due instead to activation of a gK(Ca) by the conditioning depolarization, since
facilitation of transmitter release increased monotonically with the amplitude of the
conditioning depolarization, and the early time course of the decay of facilitation
was prolonged when gK(Ca) was blocked. The different time courses for decay of the
presynaptic potential (20 ms) and facilitation (> 50 ms) suggest either that residual free calcium does not account for facilitation at the crayfish NMJ or that the
transmitter release mechanism has a markedly higher affinity or stoichiometry for
internal free calcium than does g K(Ca). Finally, our data suggest that the calcium
channels responsible for transmitter release at the crayfish NMJ are not of the L, N,
or T type.This work was partially supported by NIAAA grant AA0776 to G. D. Bittner.Neuroscienc
Automated, objective texture segmentation of multibeam echosounder data - Seafloor survey and substrate maps from James Island to Ozette Lake, Washington Outer Coast
Without knowledge of basic seafloor characteristics, the ability to address any number of critical marine and/or coastal management issues is diminished. For example,
management and conservation of essential fish habitat (EFH), a requirement mandated by federally guided fishery management plans (FMPs), requires among other things a
description of habitats for federally managed species. Although the list of attributes important to habitat are numerous, the ability to efficiently and effectively describe many, and especially at the scales required, does not exist with the tools currently available. However, several characteristics of seafloor morphology are readily obtainable at multiple scales and can serve as useful descriptors of habitat. Recent advancements in acoustic technology, such as multibeam echosounding (MBES), can provide remote indication of surficial sediment properties such as texture, hardness, or roughness, and further permit highly detailed renderings of seafloor morphology. With acoustic-based surveys providing a relatively efficient method for data acquisition, there exists a need for
efficient and reproducible automated segmentation routines to process the data. Using MBES data collected by the Olympic Coast National Marine Sanctuary (OCNMS), and
through a contracted seafloor survey, we expanded on the techniques of Cutter et al. (2003) to describe an objective repeatable process that uses parameterized local Fourier
histogram (LFH) texture features to automate segmentation of surficial sediments from acoustic imagery using a maximum likelihood decision rule. Sonar signatures and
classification performance were evaluated using video imagery obtained from a towed camera sled. Segmented raster images were converted to polygon features and attributed
using a hierarchical deep-water marine benthic classification scheme (Greene et al. 1999) for use in a geographical information system (GIS). (PDF contains 41 pages.
Hydrologic Properties of Subarctic Organic Soils
Completion Report
for
U. S. Forest Service
Institute of Northern Forestry
Cooperative Agreement No. 16 USC 581; 581a-581iThe need for understanding the natural system and how it responds
to various stresses is important; this is especially so in an environment
where the climate not only sustains permafrost, but develops
massive seasonal frost as well. Consequently, the role of the shallow
surface organic layer is also quite important. Since a slight change in
the soil thermal regime may bring about a phase change in the water or
ice, therefore, the system response to surface alterations such as
burning can be quite severe. The need for a better understanding of the
behavior and properties of the organic layer is, therefore, accentuated.
The central theme of this study was the examination of the hydrologic
and hydraulic properties of subarctic organic soils. Summarized
in this paper are the results of three aspects of subarctic organic soil
examinations conducted during the duration of the project. First, a
field site was set up in Washington Creek with the major emphasis on
measuring numerous variables of that soil system during the summer. The
greatest variations in moisture content occur in the thick organic soils
that exist at this site. Our major emphasis was to study the soil
moisture levels in these soils. This topic is covered in the first
major section, including associated laboratory studies. Those laboratory
studies include investigations of several hydraulic and hydrologic
properties of taiga organic and mineral soils. Second, some field data
on organic moisture levels was collected at the site of prescribed burns
in Washington Creek to ascertain the sustainability of fires as a function
of moisture levels. This portion of the study is described under the
second major heading. The last element of this study was a continued
application of the two-dimensional flow model that was developed in an
earlier study funded by the U. S. Forest Service, Institute of Northern
Forestry, and reported by Kane, Luthin, and Taylor (1975a).
Many of the results and concepts gathered in the field work were
integrated into the modeling effort, which is aimed at producing better
estimates of the hydrologic effects of surface disturbances in the black
spruce taiga subarctic ecosystem. This knowledge should also contribute
to better fire management decisions of the same system.The work upon which this report is based was made possible by a
cooperative aid agreement funded by the U. S. Forest Service, Institute
of Northern Forestry, Fairbanks, Alaska. Contribution to this study was
also made by Ohio State University
Transfer learning approach for financial applications
Artificial neural networks learn how to solve new problems through a
computationally intense and time consuming process. One way to reduce the
amount of time required is to inject preexisting knowledge into the network. To
make use of past knowledge, we can take advantage of techniques that transfer
the knowledge learned from one task, and reuse it on another (sometimes
unrelated) task. In this paper we propose a novel selective breeding technique
that extends the transfer learning with behavioural genetics approach proposed
by Kohli, Magoulas and Thomas (2013), and evaluate its performance on financial
data. Numerical evidence demonstrates the credibility of the new approach. We
provide insights on the operation of transfer learning and highlight the
benefits of using behavioural principles and selective breeding when tackling a
set of diverse financial applications problems
Modeling human behavior in user-adaptive systems: recent advances using soft computing techniques
Adaptive Hypermedia systems are becoming more important in our everyday activities and users are expecting more intelligent services from them. The key element of a generic adaptive hypermedia system is the user model. Traditional machine learning techniques used to create user models are usually too rigid to capture the inherent uncertainty of human behavior. In this context, soft computing techniques can be used to handle and process human uncertainty and to simulate human decision-making. This paper examines how soft computing techniques, including fuzzy logic, neural networks, genetic algorithms, fuzzy clustering and neuro-fuzzy systems, have been used, alone or in combination with other machine learning techniques, for user modeling from 1999 to 2004. For each technique, its main applications, limitations and future directions for user modeling are presented. The paper also presents guidelines that show which soft computing techniques should be used according to the task implemented by the application
Online learning : towards enabling choice
Education is rapidly evolving from an opportunity that was provided mainly for an elite to one that is available to a mass markets and as such is prone to the forces generated by this environment. Where, in the established pattern, commercial interest was limited mainly to the use of skills developed during the educational process, the future model of educational provision will involve extensive commercial activity in the production, delivery and marketing of material. Already there are a number of commercial companies offering framework products enabling "off the shelf solutions" for the construction and delivery of web based courses in any subject area. The commercialisation of education is underway and it is inevitable that it will be viewed, by entrepreneurs and customers alike, as any other commercial product. It would seem reasonable that the consumer should be able to evaluate the performance of these new modes of working in a similar manner to other commercial products. This paper draws together current thinking on the problems associated with evaluating computer and communication based learning
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