5,345 research outputs found
Reionization history constraints from neural network based predictions of high-redshift quasar continua
Observations of the early Universe suggest that reionization was complete by
, however, the exact history of this process is still unknown. One
method for measuring the evolution of the neutral fraction throughout this
epoch is via observing the Ly damping wings of high-redshift quasars.
In order to constrain the neutral fraction from quasar observations, one needs
an accurate model of the quasar spectrum around Ly, after the spectrum
has been processed by its host galaxy but before it is altered by absorption
and damping in the intervening IGM. In this paper, we present a novel machine
learning approach, using artificial neural networks, to reconstruct quasar
continua around Ly. Our QSANNdRA algorithm improves the error in this
reconstruction compared to the state-of-the-art PCA-based model in the
literature by 14.2% on average, and provides an improvement of 6.1% on average
when compared to an extension thereof. In comparison with the extended PCA
model, QSANNdRA further achieves an improvement of 22.1% and 16.8% when
evaluated on low-redshift quasars most similar to the two high-redshift quasars
under consideration, ULAS J1120+0641 at and ULAS J1342+0928 at
, respectively. Using our more accurate reconstructions of these two
quasars, we estimate the neutral fraction of the IGM using a homogeneous
reionization model and find at
and at . Our
results are consistent with the literature and favour a rapid end to
reionization
A Heterosynaptic Learning Rule for Neural Networks
In this article we intoduce a novel stochastic Hebb-like learning rule for
neural networks that is neurobiologically motivated. This learning rule
combines features of unsupervised (Hebbian) and supervised (reinforcement)
learning and is stochastic with respect to the selection of the time points
when a synapse is modified. Moreover, the learning rule does not only affect
the synapse between pre- and postsynaptic neuron, which is called homosynaptic
plasticity, but effects also further remote synapses of the pre- and
postsynaptic neuron. This more complex form of synaptic plasticity has recently
come under investigations in neurobiology and is called heterosynaptic
plasticity. We demonstrate that this learning rule is useful in training neural
networks by learning parity functions including the exclusive-or (XOR) mapping
in a multilayer feed-forward network. We find, that our stochastic learning
rule works well, even in the presence of noise. Importantly, the mean learning
time increases with the number of patterns to be learned polynomially,
indicating efficient learning.Comment: 19 page
Vertical gas accretion impacts the carbon-to-oxygen ratio of gas giant atmospheres
Recent theoretical, numerical, and observational work have suggested that
when a growing planet opens a gap in its disk the flow of gas into the gap is
dominated by gas falling vertically from a height of at least one gas scale
height. Our primary objective is to include, for the first time, the chemical
impact that accreting gas above the midplane will have on the resulting C/O. We
compute the accretion of gas onto planetary cores beginning at different disk
radii and track the chemical composition of the gas and small icy grains to
predict the resulting carbon-to-oxygen ratio (C/O) in their atmospheres. In our
model, all of the planets which began their evolution inward of 60 AU open a
gap in the gas disk, and hence are chemically affected by the vertically
accreting gas. Two important conclusions follow from this vertical flow: (1)
more oxygen rich icy dust grains become available for accretion onto the
planetary atmosphere. (2) The chemical composition of the gas dominates the
final C/O of planets in the inner ( 20 AU) part of the disk. This implies
that with the launch of the James Webb Space Telescope we can trace the disk
material that sets the chemical composition of exoplanetary atmospheres.Comment: Accepted for publication in A&
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Large Ly α opacity fluctuations and low CMB τ in models of late reionization with large islands of neutral hydrogen extending to z < 5.5
High-redshift QSO spectra show large spatial fluctuations in the Ly-alpha
opacity of the intergalactic medium on surprisingly large scales at z>~5.5. We
present a radiative transfer simulation of cosmic reionization driven by
galaxies that reproduces this large scatter and the rapid evolution of the
Ly-alpha opacity distribution at 5<z<6. The simulation also reproduces the low
Thomson scattering optical depth reported by the latest CMB measurement and is
consistent with the observed short near-zones and strong red damping wings in
the highest-redshift QSOs. It also matches the rapid disappearance of observed
Ly-alpha emission by galaxies at z>~6. Reionization is complete at z=5.3 in our
model, and 50% of the volume of the Universe is ionized at z=7. Agreement with
the Ly-alpha forest data in such a late reionization model requires a rapid
evolution of the ionizing emissivity of galaxies that peaks at z~6.8. The late
end of reionization results in a large scatter in the photoionisation rate and
the neutral hydrogen fraction at redshifts as low as z<~5.5 with large residual
neutral 'islands' that can produce very long Gunn-Peterson troughs resembling
those seen in the data
Dual role of DNA methylation inside and outside of CTCF-binding regions in the transcriptional regulation of the telomerase hTERT gene
Expression of hTERT is the major limiting factor for telomerase activity. We previously showed that methylation of the hTERT promoter is necessary for its transcription and that CTCF can repress hTERT transcription by binding to the first exon. In this study, we used electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) to show that CTCF does not bind the methylated first exon of hTERT. Treatment of telomerase-positive cells with 5-azadC led to a strong demethylation of hTERT 5′-regulatory region, reactivation of CTCF binding and downregulation of hTERT. Although complete hTERT promoter methylation was associated with full transcriptional repression, detailed mapping showed that, in telomerase-positive cells, not all the CpG sites were methylated, especially in the promoter region. Using a methylation cassette assay, selective demethylation of 110 bp within the core promoter significantly increased hTERT transcriptional activity. This study underlines the dual role of DNA methylation in hTERT transcriptional regulation. In our model, hTERT methylation prevents binding of the CTCF repressor, but partial hypomethylation of the core promoter is necessary for hTERT expression
The formation of CO through consumption of gas-phase CO on vacuum-UV irradiated water ice
[Abridged] Observations of protoplanetary disks suggest that they are
depleted in gas-phase CO. It has been posed that gas-phase CO is chemically
consumed and converted into less volatile species through gas-grain processes.
Observations of interstellar ices reveal a CO component within HO ice
suggesting co-formation. The aim of this work is to experimentally verify the
interaction of gas-phase CO with solid-state OH radicals above the sublimation
temperature of CO. Amorphous solid water (ASW) is deposited at 15 K and
followed by vacuum-UV (VUV) irradiation to dissociate HO and create OH
radicals. Gas-phase CO is simultaneously admitted and only adsorbs with a short
residence time on the ASW. Products in the solid state are studied with
infrared spectroscopy and once released into the gas phase with mass
spectrometry. Results show that gas-phase CO is converted into CO, with an
efficiency of 7-27%, when interacting with VUV irradiated ASW. Between 40 and
90 K, CO production is constant, above 90 K, O production takes over.
In the temperature range of 40-60 K, the CO remains in the solid state,
while at temperatures 70 K the formed CO is released into the gas
phase. We conclude that gas-phase CO reacts with solid-state OH radicals above
its sublimation temperature. This gas-phase CO and solid-state OH radical
interaction could explain the observed CO embedded in water-rich ices. It
may also contribute to the observed lack of gas-phase CO in planet-forming
disks, as previously suggested. Our experiments indicate a lower water ice
dissociation efficiency than originally adopted in model descriptions of
planet-forming disks and molecular clouds. Incorporation of the reduced water
ice dissociation and increased binding energy of CO on a water ice surfaces in
these models would allow investigation of this gas-grain interaction to its
full extend.Comment: Accepted for publication in Astronomy & Astrophysic
CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing
The gas-phase CO abundance (relative to hydrogen) in protoplanetary disks
decreases by up to 2 orders of magnitude from its ISM value ,
even after accounting for freeze-out and photo-dissociation. Previous studies
have shown that while local chemical processing of CO and the sequestration of
CO ice on solids in the midplane can both contribute, neither of these
processes appears capable of consistently reaching the observed depletion
factors on the relevant timescale of . In this study, we
model these processes simultaneously by including a compact chemical network
(centered on carbon and oxygen) to 2D () simulations of the outer
() disk regions that include turbulent diffusion, pebble
formation, and pebble dynamics. In general, we find that the CO/H abundance
is a complex function of time and location. Focusing on CO in the warm
molecular layer, we find that only the most complete model (with chemistry and
pebble evolution included) can reach depletion factors consistent with
observations. In the absence of pressure traps, highly-efficient planetesimal
formation, or high cosmic ray ionization rates, this model also predicts a
resurgence of CO vapor interior to the CO snowline. We show the impact of
physical and chemical processes on the elemental (C/O) and (C/H) ratios (in the
gas and ice phases), discuss the use of CO as a disk mass tracer, and, finally,
connect our predicted pebble ice compositions to those of pristine
planetesimals as found in the Cold Classical Kuiper Belt and debris disks.Comment: Accepted for publication in The Astrophysical Journa
On Comparing the Performance of Dynamic Multi-Network Optimizations
Abstract-With a large variety of wireless access technologies available, multi-homed devices may strongly improve the performance and reliability of communication when using multiple networks simultaneously. A key question for the practical application of multi-path strategies is the granularity at which the traffic streams should be dispersed among the available networks. This level of granularity may be expected to have a major impact on both the efficiency and complexity of practical realizations. Motivated by this, we compare two dynamic strategies that operate at different levels of granularity. The first strategy, which we call network selection, requires little operational complexity and dynamically assigns an arriving application data transfer to the network that delivers the highest expected performance. Our second strategy, which we call traffic-splitting, is of higher complexity and aims to optimally split individual data transfers among the available networks. To this end, we (1) develop quantitative models that describe the performance of both strategies, (2) determine the (near-)optimal algorithms for both strategies, and (3) validate the efficiency and practical usefulness of the algorithms via extensive network simulations and experiments in a real-life testbed environment. These experimental results show that the optimal strategies obtained from the theoretical models lead to extremely well-performing solutions in practical circumstances. Moreover, the results show that the splitting of data transfers, which is easy to embed in the network requiring no information on the number of flows in the system, leads to a much better performance compared to dynamic network selection
Preliminary results of a cohort study of induction chemotherapy-based treatment for locally recurrent rectal cancer
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