245 research outputs found
Foregrounds for observations of the cosmological 21 cm line: II. Westerbork observations of the fields around 3C196 and the North Celestial Pole
In the coming years a new insight into galaxy formation and the thermal
history of the Universe is expected to come from the detection of the highly
redshifted cosmological 21 cm line. The cosmological 21 cm line signal is
buried under Galactic and extragalactic foregrounds which are likely to be a
few orders of magnitude brighter. Strategies and techniques for effective
subtraction of these foreground sources require a detailed knowledge of their
structure in both intensity and polarization on the relevant angular scales of
1-30 arcmin. We present results from observations conducted with the Westerbork
telescope in the 140-160 MHz range with 2 arcmin resolution in two fields
located at intermediate Galactic latitude, centred around the bright quasar
3C196 and the North Celestial Pole. They were observed with the purpose of
characterizing the foreground properties in sky areas where actual observations
of the cosmological 21 cm line could be carried out. The polarization data were
analysed through the rotation measure synthesis technique. We have computed
total intensity and polarization angular power spectra. Total intensity maps
were carefully calibrated, reaching a high dynamic range, 150000:1 in the case
of the 3C196 field. [abridged]Comment: 20 pages, 22 figures, accepted for publication in A&A. A version with
full resolution figures is available at
http://www.astro.rug.nl/~bernardi/NCP_3C196/bernardi.pd
Prospects for detecting the 21cm forest from the diffuse intergalactic medium with LOFAR
We discuss the feasibility of the detection of the 21cm forest in the diffuse
IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been
derived using simulations of reionization which include detailed radiative
transfer of ionizing photons. We find that the spectra from reionization models
with similar total comoving hydrogen ionizing emissivity but different
frequency distribution look remarkably similar. Thus, unless the reionization
histories are very different from each other (e.g. a predominance of UV vs.
x-ray heating) we do not expect to distinguish them by means of observations of
the 21cm forest. Because the presence of a strong x-ray background would make
the detection of 21cm line absorption impossible, the lack of absorption could
be used as a probe of the presence/intensity of the x-ray background and the
thermal history of the universe. Along a random line of sight LOFAR could
detect a global suppression of the spectrum from z>12, when the IGM is still
mostly neutral and cold, in contrast with the more well-defined, albeit broad,
absorption features visible at lower redshift. Sharp, strong absorption
features associated with rare, high density pockets of gas could be detected
also at z~7 along preferential lines of sight.Comment: 12 pages, 13 figures. MNRAS, in pres
Real-time ultrasound-guided catheterisation of the internal jugular vein: a prospective comparison with the landmark technique in critical care patients
INTRODUCTION: Central venous cannulation is crucial in the management of the critical care patient. This study was designed to evaluate whether real-time ultrasound-guided cannulation of the internal jugular vein is superior to the standard landmark method. METHODS: In this randomised study, 450 critical care patients who underwent real-time ultrasound-guided cannulation of the internal jugular vein were prospectively compared with 450 critical care patients in whom the landmark technique was used. Randomisation was performed by means of a computer-generated random-numbers table, and patients were stratified with regard to age, gender, and body mass index. RESULTS: There were no significant differences in gender, age, body mass index, or side of cannulation (left or right) or in the presence of risk factors for difficult venous cannulation such as prior catheterisation, limited sites for access attempts, previous difficulties during catheterisation, previous mechanical complication, known vascular abnormality, untreated coagulopathy, skeletal deformity, and cannulation during cardiac arrest between the two groups of patients. Furthermore, the physicians who performed the procedures had comparable experience in the placement of central venous catheters (p = non-significant). Cannulation of the internal jugular vein was achieved in all patients by using ultrasound and in 425 of the patients (94.4%) by using the landmark technique (p < 0.001). Average access time (skin to vein) and number of attempts were significantly reduced in the ultrasound group of patients compared with the landmark group (p < 0.001). In the landmark group, puncture of the carotid artery occurred in 10.6% of patients, haematoma in 8.4%, haemothorax in 1.7%, pneumothorax in 2.4%, and central venous catheter-associated blood stream infection in 16%, which were all significantly increased compared with the ultrasound group (p < 0.001). CONCLUSION: The present data suggest that ultrasound-guided catheterisation of the internal jugular vein in critical care patients is superior to the landmark technique and therefore should be the method of choice in these patients
Human Coronary Artery Remodeling, Beginning and End of the Atherosclerotic Process
BACKGROUND, AIMS OF THE STUDY: The objective of the study was to relate the progress of coronary artery remodeling to the earliest stages of the atherosclerotic process. For this purpose, a mathematical model for description of dimensional change of the coronary artery wall and its constituent components was developed and applied. MATERIALS AND METHODS: The study used coronary artery samples randomly taken from each of 83 consecutive, unselected postmortems. All samples were routinely fixed and processed to paraffin for the preparation of right-angled, 5-micron sections, routinely stained and mounted for subsequent analysis. Computer assisted image analysis, using 32 systematic random, radial sampling lines, was used for interactive measurements of distance from centre of lumen to points defining intima, media and adventitia thickness along the radial intercept, which were subsequently tabled for analysis of variance, calculations of (group –vessel) means, and related to stage of pathology. RESULTS: Pre-atherosclerotic changes, before any localised changes in especially intima dimensions, are found, consisting of a process of gradual vascular widening, associated with temporally at least partly dissociated increases in width, which as a fraction of total vessel radius show a phased process. In these, the intima first increases, subsequently remains stable, and finally reduces in width proportionally to the increasing diameter. The media shows a similar initial increase, on average stabilising in the third phase after reaching a plateau value in the second. The adventitia, already increasing in phase 1, continues to increase in phase 2, accelerating in phase 3. The complex process, as found, occurs systematically in all vessels, is distributed circumferentially, and precedes the development of localised lesions of the intima. CONCLUSIONS: The findings suggest the existence of a diffuse complex of changes, consisting of a gradual vascular widening followed by narrowing, with associated mural changes reflecting the atherosclerotic process
Foregrounds for observations of the cosmological 21 cm line: I. First Westerbork measurements of Galactic emission at 150 MHz in a low latitude field
We present the first results from a series of observations conducted with the
Westerbork telescope in the 140--160 MHz range with a 2 arcmin resolution aimed
at characterizing the properties of the foregrounds for epoch of reionization
experiments. For the first time we have detected fluctuations in the Galactic
diffuse emission on scales greater than 13 arcmin at 150 MHz, in the low
Galactic latitude area known as Fan region. Those fluctuations have an of
14 K. The total intensity power spectrum shows a power--law behaviour down to
with slope . The detection of
diffuse emission at smaller angular scales is limited by residual point
sources. We measured an confusion noise of 3 mJy beam.
Diffuse polarized emission was also detected for the first time at this
frequency. The polarized signal shows complex structure both spatially and
along the line of sight. The polarization power spectrum shows a power--law
behaviour down to with slope .
The of polarization fluctuations is 7.2 K on 4 arcmin scales. By
extrapolating the measured spectrum of total intensity emission, we find a
contamination on the cosmological signal of K on 5 arcmin scales and a corresponding value
of 18.3 K at the same angular scale. The level of the polarization power
spectrum is K on 5 arcmin scales. Given its exceptionally
bright polarized signal, the Fan region is likely to represent an upper limit
on the sky brightness at moderate and high Galactic latitude.Comment: Minor corrections made to match the final version printed on A&A. A
version with high resolution figures is available at
http://www.astro.rug.nl/~bernardi/FAN/fan.pd
Foreground simulations for the LOFAR - Epoch of Reionization Experiment
Future high redshift 21-cm experiments will suffer from a high degree of
contamination, due both to astrophysical foregrounds and to non-astrophysical
and instrumental effects. In order to reliably extract the cosmological signal
from the observed data, it is essential to understand very well all data
components and their influence on the extracted signal. Here we present
simulated astrophysical foregrounds datacubes and discuss their possible
statistical effects on the data. The foreground maps are produced assuming 5
deg x 5 deg windows that match those expected to be observed by the LOFAR
Epoch-of-Reionization (EoR) key science project. We show that with the expected
LOFAR-EoR sky and receiver noise levels, which amount to ~52 mK at 150 MHz
after 300 hours of total observing time, a simple polynomial fit allows a
statistical reconstruction of the signal. We also show that the polynomial
fitting will work for maps with realistic yet idealised instrument response,
i.e., a response that includes only a uniform uv coverage as a function of
frequency and ignores many other uncertainties. Polarized galactic synchrotron
maps that include internal polarization and a number of Faraday screens along
the line of sight are also simulated. The importance of these stems from the
fact that the LOFAR instrument, in common with all current interferometric EoR
experiments has an instrumentally polarized response.Comment: 18 figures, 3 tables, accepted to be published in MNRA
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