8,492 research outputs found
Structure in the local Galactic ISM on scales down to 1 pc, from multi-band radio polarization observations
We discuss observations of the linearly polarized component of the diffuse
galactic radio background. These observations, with an angular resolution of
4', were made with the Westerbork Synthesis Radio Telescope (WSRT) in 5
frequency bands in the range 341-375 MHz. The linearly polarized intensity P
(with polarized brightness temperature going up to 10K) shows a `cloudy'
structure, with characteristic scales of 15'-30', which contains relatively
long, but very narrow `canals' (essentially unresolved) in which P is only a
small fraction of that in the neighbouring beams.
These `canals' are generally seen in more than one frequency band, although
their appearance changes between bands. They are probably due to depolarization
within the synthesized beam, because the change in polarization angle across
the deepest `canals' is in general close to 90 degrees (or 270 etc.). These
very abrupt changes in polarization angle, which are seen only across the
`canals', seem to be accompanied by abrupt changes in the Rotation Measure
(RM), which may have the right magnitude to create the difference of close to
90 degrees in polarization angle, and thereby the `canals'.
The structure in the polarization maps is most likely due to Faraday rotation
modulation of the probably smooth polarized radiation emitted in the halo of
our Galaxy by the fairly local ISM (up to 500 pc). Therefore, the abrupt
changes of RM across the `canals' provide evidence for very thin (about 1 pc),
and relatively long transition regions in the ISM, across which the RM changes
by as much as 100%. Such drastic RM changes may well be due primarily to abrupt
changes in the magnetic field.Comment: 4 pages, 4 figures, to be published in A&A Letter
Parsec-scale structure in the warm ISM from polarized galactic radio background observations
We present multi-frequency polarization observations of the diffuse radio
synchrotron background modulated by Faraday rotation, in two directions of
positive latitude. No extended total intensity I is observed, which implies
that total intensity has no structure on scales smaller than approximately a
degree. Polarized intensity and polarization angle, however, show abundant
small-scale structure on scales from arcminutes to degrees. Rotation Measure
(RM) maps show coherent structure over many synthesized beams, but also abrupt
large changes over one beam. RM's from polarized extragalactic point sources
are correlated over the field in each of the two fields, indicating a galactic
component to the RM, but show no correlation with the RM map of the diffuse
radiation. The upper limit in structure in I puts constraints on the random and
regular components of the magnetic field in the galactic interstellar medium
and halo. The emission is partly depolarized so that the observed polarization
mostly originates from a nearby part of the medium. This explains the lack of
correlation between RM from diffuse emission and from extragalactic point
sources as the latter is built up over the entire path length through the
medium.Comment: To appear in "Astrophysical Polarized Backgrounds", Conference
Proceedings, eds S. Cecchini, S. Cortiglioni, R. Sault and C. Sbarra, in
pres
Grense vir die taalwetenskap aan die hand van 'n gegewe raamwerk
In order to study a subject with all its facets will imply, in linguistics, that contexts also have to be taken into account. In order to give a full account of language as a means of communication one also has to pay attention to the use of language. This has the result that the language (speech) act has to be studied carefully by the linguist, because it involves the correlation among speaker, auditor and utterance in language-in-usage
Surrogaatmoederskap: ’n teologies-etiese beoordeling
Surrogate motherhood is the process by which a woman (the surrogate mother) conceives, carries and gives birth to a child for another woman or couple. Surrogate motherhood is related to issues such as masturbation, artificial insemination, in vitrofertilization (in vitro conception) and experimentation with embryos, which could entail the destruction or killing of embryos. Opinions about these matters would undoubtedly influence one’s perception of surrogate motherhood. Christians believe that conception in or outside the human body is made possible by God God gives life at the moment of conception. Therefore an embryo can never be considered as mere matter From a Christian point of view it is not imperative to condemn surrogate motherhood in all instances. Nevertheless, in view of what is taught in the Holy Scriptures and also because of certain practical implications, Christians cannot but be negative about the whole issue and would therefore hesitate to make use of such procedures
WSRT Faraday tomography of the Galactic ISM at \lambda \sim 0.86 m
We investigate the distribution and properties of Faraday rotating and
synchrotron emitting regions in the Galactic ISM in the direction of the
Galactic anti-centre. We apply Faraday tomography to a radio polarization
dataset that we obtained with the WSRT. We developed a new method to calculate
a linear fit to periodic data, which we use to determine rotation measures from
our polarization angle data. From simulations of a Faraday screen + noise we
could determine how compatible the data are with Faraday screens. An
unexpectedly large fraction of 14% of the lines-of-sight in our dataset show an
unresolved main component in the Faraday depth spectrum. For lines-of-sight
with a single unresolved component we demonstrate that a Faraday screen in
front of a synchrotron emitting region that contains a turbulent magnetic field
component can explain the data.Comment: 5 pages, 5 figures. Accepted for publication as a Letter to the
Editor in A&
Motion of a sphere through an aging system
We have investigated the drag on a sphere falling through a clay suspension
that has a yield stress and exhibits rheological aging. The drag force
increases with both speed and the rest time between preparation of the system
and the start of the experiment, but there exists a nonzero minimum speed below
which steady motion is not possible. We find that only a very thin layer of
material around the sphere is fluidized when it moves, while the rest of
suspension is deformed elastically. This is in marked contrast to what is found
for yield-stress fluids that do not age.Comment: latex, 4 figure
The radio source B 1834+620: A double-double radio galaxy with interesting properties
We present a study of the peculiar radio galaxy B 1834+620. It is
characterised by the presence of a 420-kpc large edge-brightened radio source
which is situated within, and well aligned with, a larger (1.66 Mpc) radio
source. Both sources apparently originate in the same host galaxy, which has a
R_s-magnitude of 19.7 and a redshift of 0.5194, as determined from the strong
emission-lines in the spectrum. We have determined the rotation measures
towards this source, as well as the radio spectral energy distribution of its
components. The radio spectrum of the large outer source is steeper than that
of the smaller inner source. The radio core has a spectrum that peaks at a
frequency of a few GHz. The rotation measures towards the four main components
are quite similar, within rad m of 58 rad m. They are
probably largely galactic in origin. We have used the presence of a bright
hotspot in the northern outer lobe to constrain the advance velocity of the
inner radio lobes to the range between 0.19c and 0.29c, depending on the
orientation of the source. This corresponds to an age of this structure in the
range between 2.6 and 5.8 Myr. We estimate a density of the ambient medium of
the inner lobes of \la 1.6 \times 10^{-30} gr\,cm (particle density
\la 8 \times 10^{-7} cm). A low ambient density is further supported
by the discrepancy between the large optical emission-line luminosity of the
host galaxy and the relatively low radio power of the inner lobes.Comment: Accepted for publication in MNRA
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