2,360 research outputs found
Improved Neural Relation Detection for Knowledge Base Question Answering
Relation detection is a core component for many NLP applications including
Knowledge Base Question Answering (KBQA). In this paper, we propose a
hierarchical recurrent neural network enhanced by residual learning that
detects KB relations given an input question. Our method uses deep residual
bidirectional LSTMs to compare questions and relation names via different
hierarchies of abstraction. Additionally, we propose a simple KBQA system that
integrates entity linking and our proposed relation detector to enable one
enhance another. Experimental results evidence that our approach achieves not
only outstanding relation detection performance, but more importantly, it helps
our KBQA system to achieve state-of-the-art accuracy for both single-relation
(SimpleQuestions) and multi-relation (WebQSP) QA benchmarks.Comment: Accepted by ACL 2017 (updated for camera-ready
Where can we really find the First Stars' Remnants today?
A number of recent numerical investigations concluded that the remnants of
rare structures formed at very high redshift, such as the very first stars and
bright redshift z~6 QSOs, are preferentially located at the center of the most
massive galaxy clusters at redshift z=0. In this paper we readdress this
question using a combination of cosmological simulations of structure formation
and extended Press-Schechter formalism and we show that the typical remnants of
Population III stars are instead more likely to be found in a group
environment, that is in dark matter halos of mass ~2x10^{13} h^{-1}M_sun.
Similarly, the descendants of the brightest z~6 QSOs are expected to be in
medium-sized clusters (mass of a few 10^{14} h^{-1}M_sun), rather than in the
most massive superclusters (M>10^{15} h^{-1}M_sun) found within the typical 1
Gpc^3 cosmic volume where a bright z~6 QSO lives. The origin of past claims
that the most massive clusters preferentially host these remnants is rooted in
the numerical method used to initialize their numerical simulations: Only a
small region of the cosmological volume of interest was simulated with
sufficient resolution to identify low-mass halos at early times, and this
region was chosen to host the most massive halo in the cosmological volume at
late times. The conclusion that the earliest structures formed in the entire
cosmological volume evolve into the most massive halo at late times was thus
arrived at by construction. We demonstrate that, to the contrary, the first
structures to form in a cosmological region evolve into relatively typical
objects at later times. We propose alternative numerical methods for simulating
the earliest structures in cosmological volumes.Comment: 18 pages, 5 figures, ApJ accepted, high resolution version of the
paper available at http://www.stsci.edu/~trenti/papers/halo_evolution.pd
Small-scale CMB Temperature and Polarization Anisotropies due to Patchy Reionization
We study contributions from inhomogeneous (patchy) reionization to arcminute
scale () cosmic microwave background (CMB) anisotropies.
We show that inhomogeneities in the ionization fraction, rather than in the
mean density, dominate both the temperature and the polarization power spectra.
Depending on the ionization history and the clustering bias of the ionizing
sources, we find that rms temperature fluctuations range from 2 K to 8
K and the corresponding values for polarization are over two orders of
magnitude smaller. Reionization can significantly bias cosmological parameter
estimates and degrade gravitational lensing potential reconstruction from
temperature maps but not from polarization maps. We demonstrate that a simple
modeling of the reionization temperature power spectrum may be sufficient to
remove the parameter bias. The high- temperature power spectrum will
contain some limited information about the sources of reionization.Comment: 11 pages, 8 figures. Minor changes to match version accepted by Ap
Fast Large Volume Simulations of the 21 cm Signal from the Reionization and pre-Reionization Epochs
While limited to low spatial resolution, the next generation low-frequency
radio interferometers that target 21 cm observations during the era of
reionization and prior will have instantaneous fields-of-view that are many
tens of square degrees on the sky. Predictions related to various statistical
measurements of the 21 cm brightness temperature must then be pursued with
numerical simulations of reionization with correspondingly large volume box
sizes, of order 1000 Mpc on one side. We pursue a semi-numerical scheme to
simulate the 21 cm signal during and prior to Reionization by extending a
hybrid approach where simulations are performed by first laying down the linear
dark matter density field, accounting for the non-linear evolution of the
density field based on second-order linear perturbation theory as specified by
the Zel'dovich approximation, and then specifying the location and mass of
collapsed dark matter halos using the excursion-set formalism. The location of
ionizing sources and the time evolving distribution of ionization field is also
specified using an excursion-set algorithm. We account for the brightness
temperature evolution through the coupling between spin and gas temperature due
to collisions, radiative coupling in the presence of Lyman-alpha photons and
heating of the intergalactic medium, such as due to a background of X-ray
photons. The hybrid simulation method we present is capable of producing the
required large volume simulations with adequate resolution in a reasonable time
so a large number of realizations can be obtained with variations in
assumptions related to astrophysics and background cosmology that govern the 21
cm signal.Comment: 14 pages and 15 figures. New version to match accepted version for
MNRAS. Code available in: http://www.SimFast21.or
Probing the first galaxies with the SKA
Observations of anisotropies in the brightness temperature of the 21 cm line
of neutral hydrogen from the period before reionization would shed light on the
dawn of the first stars and galaxies. In this paper, we use large-scale
semi-numerical simulations to analyse the imprint on the 21 cm signal of
spatial fluctuations in the Lyman-alpha flux arising from the clustering of the
first galaxies. We show that an experiment such as the Square Kilometer Array
(SKA) can probe this signal at the onset of reionization, giving us important
information about the UV emission spectra of the first stars and characterizing
their host galaxies. SKA-pathfinders with ~ 10% of the full collecting area
should be capable of making a statistical detection of the 21 cm power spectrum
at redshifts z 67 MHz). We then show
that the SKA should be able to measure the three dimensional power spectrum as
a function of the angle with the line of sight and discuss the use of the
redshift space distortions as a way to separate out the different components of
the 21 cm power spectrum. We demonstrate that, at least on large scales where
the Lyman-alpha fluctuations are linear, they can be used as a model
independent way to extract the power spectra due to these Lyman-alpha
fluctuations.Comment: 13 pages, 17 figures. New version to match version accepted by A&A.
Improved discussions on the Lyman-alpha simulation, adiabatic cooling
fluctuations, the Fisher matrix approach and the Poisson term calculation.
New version of the code available at: http://www.SimFast21.or
Contribution of Cross-Correlations to the 21cm Angular Power Spectrum in the Epoch of Reionization
Measurement of the 21cm hyperfine transition of neutral hydrogen provides a
unique probe of the epoch of reionization and the Dark Ages. Three major
mechanisms are believed to dominate the radiation process: emission from
neutral hydrogen surrounding the ionized bubbles of first galaxies and/or
quasars, emission from neutral hydrogen inside minihalos, and absorption of
diffuse neutral hydrogen against the cosmic microwave background. In the
present work, by simply combining the existing analytic models for the three
mechanisms, we investigate the contribution of cross-correlation between these
three components to the total 21cm angular power spectrum, in the sense that
neutral hydrogen associated with different radiation processes traces the
large-scale structures of underlying density perturbations. While the overall
21cm power spectrum remains almost unchanged with the inclusion of the
cross-correlations, the cross-correlation may play a key role in the
determination of the 21cm power spectrum during the transition of 21cm
radiation from emission-dominated phase to absorption-dominated phase at
redshift z~20. A significant suppression in the 21cm angular power spectrum
during this transition is anticipated as the result of negative contribution of
the cross-correlation between the absorption of diffuse neutral hydrogen and
the emission components. Therefore, an accurate prediction of the cosmic 21cm
power spectrum should take the cross-correlation into account especially at the
transition phase.Comment: 10 pages, 4 figures, accepted for publication in MNRA
Counterfactual thinking in cooperation dynamics
Counterfactual Thinking is a human cognitive ability studied in a wide
variety of domains. It captures the process of reasoning about a past event
that did not occur, namely what would have happened had this event occurred,
or, otherwise, to reason about an event that did occur but what would ensue had
it not. Given the wide cognitive empowerment of counterfactual reasoning in the
human individual, the question arises of how the presence of individuals with
this capability may improve cooperation in populations of self-regarding
individuals. Here we propose a mathematical model, grounded on Evolutionary
Game Theory, to examine the population dynamics emerging from the interplay
between counterfactual thinking and social learning (i.e., individuals that
learn from the actions and success of others) whenever the individuals in the
population face a collective dilemma. Our results suggest that counterfactual
reasoning fosters coordination in collective action problems occurring in large
populations, and has a limited impact on cooperation dilemmas in which
coordination is not required. Moreover, we show that a small prevalence of
individuals resorting to counterfactual thinking is enough to nudge an entire
population towards highly cooperative standards.Comment: 18 page
Signatures of reionization on Lyman alpha emitters
We use a semi-analytic model of Lyman alpha emitters (LAEs) to constrain the
reionization history. By considering two physically motivated scenarios in
which reionization ends either early (ERM, z_i ~ 7) or late (LRM, z_i ~ 6), we
fix the global value of the IGM neutral fraction (e.g. chi_{HI}=3 times
10^{-4}, 0.15 at z=6.56 for the ERM and LRM, respectively) leaving only the
star formation efficiency and the effective escape fraction of Lya photons as
free parameters. The ERM fits the observed LAE luminosity function (LF) at
z=5.7 and 6.56 requiring no redshift evolution or mass dependence of the star
formation efficiency, and LAE star formation rates (SFR) of 3-103 solar
masses/year, contributing approximately 8% of the cosmic SFR density at z=5.7.
The LRM requires a physically uncomfortable drop of approximately 4.5 times in
the SFR of the emitters from z=6.5 to 5.7. Thus, the data seem to imply that
the Universe was already highly ionized at z=6.56. The mass-dependent Lya
transmissivity is between 0.36-0.51 (ERM) and less than 0.26 (LRM) at z=6.56.
The LF data at z=4.5 imply an extra Lya line damping factor of approximately
0.25 possibly due to dust; the presence of a (clumpy) dust component with
E(B-V) ~ 0.28 is also required to reproduce the observed large Lya equivalent
widths at the same redshift. Additional useful information can be extracted
from the line profile (weighted) skewness, found to be S_W=10-17 Angstrom for
the two reionization models, which shows an interesting L_alpha-chi_{HI}
anti-correlation, holding under the model assumptions. The shortcomings of the
model and strategies to overcome them are discussed.Comment: 15 pages, 10 figures, Accepted to MNRA
Measurements of the diffuse Galactic synchrotron spectral index and curvature from MeerKLASS pilot data
21cm intensity mapping experiments are bringing an influx of high spectral
resolution observational data in the MHz GHz regime. We use
pilot MHz data from MeerKAT in single-dish mode, recently used to
test the calibration and data reduction scheme of the upcoming MeerKLASS
survey, to probe the spectral index of diffuse synchrotron emission below 1 GHz
within , .
Through comparisons with data from the OVRO Long Wavelength Array and the Maipu
and MU surveys, we find an average spectral index of
between 45 and 1055 MHz. By fitting for spectral curvature with a spectral
index of the form , we measure
and within our target field. Our
results are in good agreement (within ) with existing measurements
from experiments such as ARCADE2 and EDGES. These results show the calibration
accuracy of current data and demonstrate that MeerKLASS will also be capable of
achieving a secondary science goal of probing the interstellar medium.Comment: 17 pages, 13 figures, accepted for publication in MNRAS. Updated to
match published paper (additional references and acknowledgements
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