1,086 research outputs found
Loss-of-function analysis of EphA receptors in retinotectal mapping
Peer reviewedPublisher PD
Two-Dimensional Synthetic Aperture Radiometry over Land Surface During Soil Moisture Experiment in 2003 (SMEX03)
Microwave radiometry at low frequencies (L-band, approx. 1.4 GHz) has been known as an optimal solution for remote sensing of soil moisture. However, the antenna size required to achieve an appropriate resolution from space has limited the development of spaceborne L-band radiometers. This problem can be addressed by interferometric technology called aperture synthesis. The Soil Moisture and Ocean Salinity (SMOS) mission will apply this technique to monitor global-scale surface parameters in the near future. The first airborne experiment using an aircraft prototype of this approach, the Two-Dimensional Synthetic Aperture Radiometer (2D-STAR), was performed in the Soil Moisture Experiment in 2003 (SMEX03). The L-band brightness temperature data acquired in Alabama by the 2DSTAR was compared with ground-based measurements of soil moisture and with C-band data collected by the Polarimetric Scanning Radiometer (PSR). Our results demonstrate a good response of the 2D-STAR brightness temperature to changes in surface wetness, both in agricultural and forest lands. The behavior of the horizontally polarized brightness temperature data with increasing view-angle over the forest area was noticeably different than over bare soil. The results from the comparison of 2D-STAR and PSR indicate a better response of the 2D-STAR to the surface wetness under both wet and dry conditions. Our results have important implications for the performance of the future SMOS mission
On the spin-temperature evolution during the epoch of reionization
Simulations estimating the differential brightness temperature of the
redshifted 21-cm from the epoch of reionization (EoR) often assume that the
spin temperature is decoupled from the background CMB temperature and is much
larger than it. Although a valid assumption towards the latter stages of the
reionization process, it does not necessarily hold at the earlier epochs.
Violation of this assumption will lead to fluctuations in differential
brightness temperature that are neither driven by density fluctuations nor by
HII regions. Therefore, it is vital to calculate the spin temperature
self-consistently by treating the Lyman-alpha and collisional coupling of spin
temperature to the kinetic temperature. In this paper we develop an extension
to the BEARS algorithm, originally developed to model reionization history, to
include these coupling effects. Here we simulate the effect in ionization and
heating for three models in which the reionization is driven by stars, miniqsos
or a mixture of both.We also perform a number of statistical tests to quantify
the imprint of the self-consistent inclusion of the spin temperature decoupling
from the CMB. We find that the evolution of the spin temperature has an impact
on the measured signal specially at redshifts higher than 10 and such evolution
should be taken into account when one attempts to interpret the observational
data.Comment: 15 pages, 18 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The Opacity of the Intergalactic Medium During Reionization: Resolving Small-Scale Structure
Early in the reionization process, the intergalactic medium (IGM) would have
been quite inhomogeneous on small scales, due to the low Jeans mass in the
neutral IGM and the hierarchical growth of structure in a cold dark matter
Universe. This small-scale structure acted as an important sink during the
epoch of reionization, impeding the progress of the ionization fronts that
swept out from the first sources of ionizing radiation. Here we present results
of high-resolution cosmological hydrodynamics simulations that resolve the
cosmological Jeans mass of the neutral IGM in representative volumes several
Mpc across. The adiabatic hydrodynamics we follow are appropriate in an
unheated IGM, before the gas has had a chance to respond to the photoionization
heating. Our focus is determination of the resolution required in cosmological
simulations in order to sufficiently sample and resolve small-scale structure
regulating the opacity of an unheated IGM. We find that a dark matter particle
mass of m_dm 1 Mpc are required. With our
converged results we show how the mean free path of ionizing radiation and
clumping factor of ionized hydrogen depends upon the ultraviolet background
(UVB) flux and redshift. We find, for example at z = 10, clumping factors
typically of 10 to 20 for an ionization rate of Gamma ~ 0.3 - 3 x 1e-12 s^-1,
with corresponding mean free paths of ~ 3 - 15 Mpc, extending previous work on
the evolving mean free path to considerably smaller scales and earlier times.Comment: Accepted for publication in the Astrophysical Journa
Heating of the Intergalactic Medium by Primordial Miniquasars
A simple analytical model is used to calculate the X-ray heating of the IGM
for a range of black hole masses. This process is efficient enough to decouple
the spin temperature of the intergalactic medium from the cosmic microwave
background (CMB) temperature and produce a differential brightness temperature
of the order of out to distances as large as a few
co-moving Mpc, depending on the redshift, black hole mass and lifetime. We
explore the influence of two types of black holes, those with and without
ionising UV radiation. The results of the simple analytical model are compared
to those of a full spherically symmetric radiative transfer code. Two simple
scenarios are proposed for the formation and evolution of black hole mass
density in the Universe. The first considers an intermediate mass black hole
that form as an end-product of Population III stars, whereas the second
considers super-massive black holes that form directly through the collapse of
massive halos with low spin parameter. These scenarios are shown not to violate
any of the observational constraints, yet produce enough X-ray photons to
decouple the spin-temperature from that of the CMB. This is an important issue
for future high redshift 21 cm observations.Comment: Replaced with a revised version to match the MNRAS accepted versio
Constraining the epoch of reionization with the variance statistic: simulations of the LOFAR case
Several experiments are underway to detect the cosmic redshifted 21-cm signal
from neutral hydrogen from the Epoch of Reionization (EoR). Due to their very
low signal-to-noise ratio, these observations aim for a statistical detection
of the signal by measuring its power spectrum. We investigate the extraction of
the variance of the signal as a first step towards detecting and constraining
the global history of the EoR. Signal variance is the integral of the signal's
power spectrum, and it is expected to be measured with a high significance. We
demonstrate this through results from a simulation and parameter estimation
pipeline developed for the Low Frequency Array (LOFAR)-EoR experiment. We show
that LOFAR should be able to detect the EoR in 600 hours of integration using
the variance statistic. Additionally, the redshift () and duration
() of reionization can be constrained assuming a parametrization. We
use an EoR simulation of and to test the
pipeline. We are able to detect the simulated signal with a significance of 4
standard deviations and extract the EoR parameters as and in 600 hours,
assuming that systematic errors can be adequately controlled. We further show
that the significance of detection and constraints on EoR parameters can be
improved by measuring the cross-variance of the signal by cross-correlating
consecutive redshift bins.Comment: 13 pages, 14 figures, Accepted for publication in MNRA
Detection and extraction of signals from the epoch of reionization using higher-order one-point statistics
Detecting redshifted 21-cm emission from neutral hydrogen in the early Universe promises to give direct constraints on the epoch of reionization (EoR). It will, though, be very challenging to extract the cosmological signal (CS) from foregrounds and noise which are orders of magnitude larger. Fortunately, the signal has some characteristics which differentiate it from the foregrounds and noise, and we suggest that using the correct statistics may tease out signatures of reionization. We generate mock data cubes simulating the output of the Low Frequency Array (LOFAR) EoR experiment. These cubes combine realistic models for Galactic and extragalactic foregrounds and the noise with three different simulations of the CS. We fit out the foregrounds, which are smooth in the frequency direction, to produce residual images in each frequency band. We denoise these images and study the skewness of the one-point distribution in the images as a function of frequency. We find that, under sufficiently optimistic assumptions, we can recover the main features of the redshift evolution of the skewness in the 21-cm signal. We argue that some of these features ¿ such as a dip at the onset of reionization, followed by a rise towards its later stages ¿ may be generic, and give us a promising route to a statistical detection of reionization
Fast Large-Scale Reionization Simulations
We present an efficient method to generate large simulations of the Epoch of
Reionization (EoR) without the need for a full 3-dimensional radiative transfer
code. Large dark-matter-only simulations are post-processed to produce maps of
the redshifted 21cm emission from neutral hydrogen. Dark matter haloes are
embedded with sources of radiation whose properties are either based on
semi-analytical prescriptions or derived from hydrodynamical simulations. These
sources could either be stars or power-law sources with varying spectral
indices. Assuming spherical symmetry, ionized bubbles are created around these
sources, whose radial ionized fraction and temperature profiles are derived
from a catalogue of 1-D radiative transfer experiments. In case of overlap of
these spheres, photons are conserved by redistributing them around the
connected ionized regions corresponding to the spheres. The efficiency with
which these maps are created allows us to span the large parameter space
typically encountered in reionization simulations. We compare our results with
other, more accurate, 3-D radiative transfer simulations and find excellent
agreement for the redshifts and the spatial scales of interest to upcoming 21cm
experiments. We generate a contiguous observational cube spanning redshift 6 to
12 and use these simulations to study the differences in the reionization
histories between stars and quasars. Finally, the signal is convolved with the
LOFAR beam response and its effects are analyzed and quantified. Statistics
performed on this mock data set shed light on possible observational strategies
for LOFAR.Comment: 18 pages, 21 figures, submitted to MNRAS For high-resolution images
follow "http://www.astro.rug.nl/~thomas/eormap.pdf
Time-evolution of ionization and heating around first stars and miniquasars
A one dimensional radiative transfer code is developed to track the
ionization and heating pattern around the first miniquasars and Population III
stars. The code follows the evolution of the ionization of the species of
hydrogen and helium and the intergalactic medium temperature profiles as a
function of redshift. The radiative transfer calculations show that the
ionization signature of the first miniquasars and stars is very similar yet the
heating pattern around the two is very different. Furthermore, the first
massive miniquasars (~>10^5 M_{sun}) do produce large ionized bubbles around
them, which can potentially be imaged directly using future radio telescopes.
It is also shown that the ionized bubbles not only stay ionized for
considerable time after the switching off of the source, but continue to expand
for a short while due to secondary collisions prompted by the X-ray part of
their spectra. Varying spectral shapes also produced sizable variations in
ionized fraction and temperature profile. We also compare the radiative
transfer results with the analytical approximation usually adopted for heating
by miniquasars and find that, because of the inadequate treatment of the He
species, the analytical approach leads to an underestimation of the temperature
in the outer radii by a factor ~5. Population III stars - with masses in the
range of 10 - 1000 M_{sun} and modelled as blackbodies at a temperature of
50000 K - are found to be efficient in ionizing their surroundings.
Observational effects on the 21 cm brightness temperature, the thermal and
kinetic Sunyaev-Ze'ldovich effects, are also studied in the context of the
upcoming radio and microwave telescopes like LOFAR and SPT.Comment: 19 pages, 24 figures, accepted to be published in MNRAS Typos in
formula 1,2 and 21 fixed. Figure 11 caption and Figure 13 change
Non-parametric foreground subtraction for 21cm epoch of reionization experiments
An obstacle to the detection of redshifted 21cm emission from the epoch of
reionization (EoR) is the presence of foregrounds which exceed the cosmological
signal in intensity by orders of magnitude. We argue that in principle it would
be better to fit the foregrounds non-parametrically - allowing the data to
determine their shape - rather than selecting some functional form in advance
and then fitting its parameters. Non-parametric fits often suffer from other
problems, however. We discuss these before suggesting a non-parametric method,
Wp smoothing, which seems to avoid some of them. After outlining the principles
of Wp smoothing we describe an algorithm used to implement it. We then apply Wp
smoothing to a synthetic data cube for the LOFAR EoR experiment. The
performance of Wp smoothing, measured by the extent to which it is able to
recover the variance of the cosmological signal and to which it avoids leakage
of power from the foregrounds, is compared to that of a parametric fit, and to
another non-parametric method (smoothing splines). We find that Wp smoothing is
superior to smoothing splines for our application, and is competitive with
parametric methods even though in the latter case we may choose the functional
form of the fit with advance knowledge of the simulated foregrounds. Finally,
we discuss how the quality of the fit is affected by the frequency resolution
and range, by the characteristics of the cosmological signal and by edge
effects.Comment: 15 pages, 12 figures; lengthened and two figures added, to match
version accepted by MNRA
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