16 research outputs found
Cosmological 21cm experiments: Searching for a needle in a haystack
There are several planned and ongoing experiments designed to explore the
Epoch of Reionization (EoR), the pivotal period during which the gas in the
intergalactic medium went from being entirely neutral to almost entirely
ionized. These experiments will probe the EoR, through the redshifted 21 cm
line from neutral hydrogen, using radio arrays: e.g. Low Frequency Array
(LOFAR) and Murchinson Widefield Array (MWA). Unfortunately however, the
cosmological 21 cm signal is highly contaminated by astrophysical foregrounds
and by non-astrophysical and instrumental effects. Therefore, to reliably
detect the cosmological signal, it is essential to understand very well all
data components, their influence on the desired signal and explore additional
complementary or corroborating probes of the EoR. These proceedings give an
overview of observational constrains of the foregrounds, present theoretical
efforts to model the foregrounds, and discuss a problem of the foreground
removal. The major results are presented for the LOFAR-EoR experiment.Comment: 12 pages, 6 figures, accepted for publication in proceedings of
"ISKAF2010 Science Meeting", PoS(ISKAF2010)02
SKA - EoR correlations and cross-correlations: kSZ, radio galaxies, and NIR background
The Universe's Cosmic Dawn (CD) and Epoch of Reionization (EoR) can be
studied using a number of observational probes that provide complementary or
corroborating information. Each of these probes suffers from its own systematic
and statistical uncertainties. It is therefore useful to consider the mutual
information that these data sets contain. In this paper, we discuss a potential
of cross-correlations between the SKA cosmological 21 cm data with: (i) the
kinetic Sunyaev- Zel'dovich (kSZ) effect in the CMB data; (ii) the galaxy
surveys; and (iii) near infrared (NIR) backgrounds.Comment: Accepted for publication in the SKA Science Book 'Advancing
Astrophysics with the Square Kilometre Array', to appear in 201
Extended X-ray emission from non-thermal sources in the COSMOS field: A detailed study of a large radio galaxy at z=1.168
X-ray selected galaxy group samples are usually generated by searching for
extended X- ray sources that reflect the thermal radiation of the intragroup
medium. On the other hand, large radio galaxies that regularly occupy galaxy
groups also emit in the X-ray window, and their contribution to X-ray selected
group samples is still not well understood. In order to investigate their
relative importance, we have carried out a systematic search for non-thermal
extended X-ray sources in the COSMOS field. Based on the morphological
coincidence of X-ray and radio extensions, out of 60 radio galaxies, and \sim
300 extended X-ray sources, we find only one candidate where the observed
extended X-ray emission arises from non- thermal processes related to radio
galaxies. We present a detailed analysis of this source, and its environment.
Our results yield that external Inverse Compton emission of the lobes is the
dominant process that generates the observed X-ray emission of our extended
X-ray candidate, with a minor contribution from the gas of the galaxy group
hosting the radio galaxy. Finally, we show that finding only one potential
candidate in the COSMOS field (in a redshift range 0 < z < 6 and with radio
luminosity between 1025 and 1030 W/Hz) is consistent with expected X-ray-counts
arising from synchrotron lobes. This implies that these sources are not a
prominent source of contamination in samples of X-ray selected clusters/groups,
but they could potentially dominate the z > 1 cluster counts at the bright end
(S_X > 7 \cdot 10^-15 erg s^-1 cm^2).Comment: 11 pages, 10 figures, 2 tables, accepted for publication in MNRA
The VLA-COSMOS Survey: V. 324 MHz continuum observations
We present 90 cm VLA imaging of the COSMOS field, comprising a circular area
of 3.14 square degrees at 8.0"x6.0" angular resolution with an average rms of
0.5 mJy/beam. The extracted catalog contains 182 sources (down to 5.5sigma), 30
of which are multi-component sources. Using Monte Carlo artificial source
simulations we derive the completeness of the catalog, and we show that our 90
cm source counts agree very well with those from previous studies. Using X-ray,
NUV-NIR and radio COSMOS data to investigate the population mix of our 90 cm
radio sample, we find that our sample is dominated by active galactic nuclei
(AGN). The average 90-20 cm spectral index (S_nu~nu**alpha, where S_nu is the
flux density at frequency nu, and alpha the spectral index) of our 90 cm
selected sources is -0.70, with an interquartile range of -0.90 to -0.53. Only
a few ultra-steep-spectrum sources are present in our sample, consistent with
results in the literature for similar fields. Our data do not show clear
steepening of the spectral index with redshift. Nevertheless, our sample
suggests that sources with spectral indices steeper than -1 all lie at z>1, in
agreement with the idea that ultra-steep-spectrum radio sources may trace
intermediate-redshift galaxies (z>1).Comment: 10 pages, 12 figures, accepted for publication in MNRA
Detectability of the 21 cm-CMB cross-correlation from the EoR
The 21-cm line fluctuations and the cosmic microwave background (CMB) are
powerful probes of the epoch of reionisation of the universe. We study the
potential of the cross-correlation between 21-cm line fluctuations and CMB
anisotropy to obtain further constraints on the reionisation history. We
compute analytically the 21-cm cross-correlation with the CMB temperature
anisotropy and polarisation, and we calculate the signal-to-noise (SN) ratio
for its detection with Planck together with LOFAR, MWA and SKA. We find, on the
one hand, that the 21-cm cross-correlation signal with CMB polarisation from
the instant reionisation can be detected with an SN ratio of for LOFAR
and for SKA. On the other hand, we confirm that the detection of the
21-cm cross-correlation with CMB polarisation is practically infeasible.Comment: 12 figure
The scale of the problem:Recovering images of reionization with Generalized Morphological Component Analysis
The accurate and precise removal of 21-cm foregrounds from Epoch of
Reionization redshifted 21-cm emission data is essential if we are to gain
insight into an unexplored cosmological era. We apply a non-parametric
technique, Generalized Morphological Component Analysis or GMCA, to simulated
LOFAR-EoR data and show that it has the ability to clean the foregrounds with
high accuracy. We recover the 21-cm 1D, 2D and 3D power spectra with high
accuracy across an impressive range of frequencies and scales. We show that
GMCA preserves the 21-cm phase information, especially when the smallest
spatial scale data is discarded. While it has been shown that LOFAR-EoR image
recovery is theoretically possible using image smoothing, we add that wavelet
decomposition is an efficient way of recovering 21-cm signal maps to the same
or greater order of accuracy with more flexibility. By comparing the GMCA
output residual maps (equal to the noise, 21-cm signal and any foreground
fitting errors) with the 21-cm maps at one frequency and discarding the smaller
wavelet scale information, we find a correlation coefficient of 0.689, compared
to 0.588 for the equivalently smoothed image. Considering only the central 50%
of the maps, these coefficients improve to 0.905 and 0.605 respectively and we
conclude that wavelet decomposition is a significantly more powerful method to
denoise reconstructed 21-cm maps than smoothing.Comment: 13 pages, 12 figures, accepted by MNRA
Initial LOFAR observations of epoch of reionization windows: II. diffuse polarized emission in the ELAIS-N1 field
Aims. This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications for extracting of the cosmological 21 cm signal from the LOw-Frequency ARray-Epoch of Reionization (LOFAR-EoR) data. Methods. We used the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes. Results. The brightness temperature of the detected Galactic emission is on average ~4 K in polarized intensity and covers the range from-10 to + 13 rad m-2 in Faraday depth. The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to 1.5%. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies Conclusions. The wide frequency range, high angular resolution, and high sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of ~1-2 rad m-2 in Faraday depth. The different polarized patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarized foregrounds is a serious complication for epoch of reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%
Power spectrum extraction for redshifted 21-cm Epoch of Reionization experiments: the LOFAR case
One of the aims of the Low Frequency Array (LOFAR) Epoch of Reionization (EoR) project is to measure the power spectrum of variations in the intensity of redshifted 21-cm radiation from the EoR. The sensitivity with which this power spectrum can be estimated depends on the level of thermal noise and sample variance, and also on the systematic errors arising from the extraction process, in particular from the subtraction of foreground contamination. We model the extraction process using realistic simulations of the cosmological signal, the foregrounds and noise, and so estimate the sensitivity of the LOFAR EoR experiment to the redshifted 21-cm power spectrum. Detection of emission from the EoR should be possible within 360 h of observation with a single station beam. Integrating for longer, and synthesizing multiple station beams within the primary (tile) beam, then enables us to extract progressively more accurate estimates of the power at a greater range of scales and redshifts. We discuss different observational strategies which compromise between depth of observation, sky coverage and frequency coverage. A plan in which lower frequencies receive a larger fraction of the time appears to be promising. We also study the nature of the bias which foreground fitting errors induce on the inferred power spectrum and discuss how to reduce and correct for this bias. The angular and line-of-sight power spectra have different merits in this respect, and we suggest considering them separately in the analysis of LOFAR data