1,319 research outputs found
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
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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
HI anisotropies associated with radio-polarimetric filaments. Steep power spectra associated with cold gas
LOFAR detected toward 3C 196 linear polarization structures which were found
subsequently to be closely correlated with cold filamentary HI structures. The
derived direction-dependent HI power spectra revealed marked anisotropies for
narrow ranges in velocity, sharing the orientation of the magnetic field as
expected for magneto hydrodynamical turbulence. Using the Galactic portion of
the Effelsberg-Bonn HI Survey we continue our study of such anisotropies in the
HI distribution in direction of two WSRT fields, Horologium and Auriga; both
are well known for their prominent radio-polarimetric depolarization canals. At
349 MHz the observed pattern in total intensity is insignificant but polarized
intensity and polarization angle show prominent ubiquitous structures with so
far unknown origin. Apodizing the HI survey data by applying a rotational
symmetric 50 percent Tukey window, we derive average and position angle
dependent power spectra. We fit power laws and characterize anisotropies in the
power distribution. We use a Gaussian analysis to determine relative abundances
for the cold and warm neutral medium. For the analyzed radio-polarimetric
targets significant anisotropies are detected in the HI power spectra; their
position angles are aligned to the prominent depolarization canals, initially
detected by WSRT. HI anisotropies are associated with steep power spectra.
Steep power spectra, associated with cold gas, are detected also in other
fields. Radio-polarimetric depolarization canals are associated with
filamentary HI structures that belong to the cold neutral medium (CNM).
Anisotropies in the CNM are in this case linked to a steepening of the
power-spectrum spectral index, indicating that phase transitions in a turbulent
medium occur on all scales. Filamentary HI structures, driven by thermal
instabilities, and radio-polarimetric filaments are associated with each other.Comment: Accepted for publication by A&A, 28 pages, 41 figures, minor updates
in styl
Low frequency measurements of synchrotron absorbing HII regions and modeling of observed synchrotron emissivity
Cosmic rays (CRs) and magnetic fields are dynamically important components in
the Galaxy, and their energy densities are comparable to that of the turbulent
interstellar gas. The interaction of CRs and Galactic magnetic fields produces
synchrotron radiation clearly visible in the radio regime. Detailed
measurements of synchrotron radiation averaged over the line-of-sight (LOS),
so-called synchrotron emissivities, can be used as a tracer of the CR density
and Galactic magnetic field (GMF) strength. Our aim is to model the synchrotron
emissivity in the Milky Way using a 3 dimensional dataset instead of
LOS-integrated intensity maps on the sky. Using absorbed HII regions we can
measure the synchrotron emissivity over a part of the LOS through the Galaxy,
changing from a 2 dimensional to a 3 dimensional view. Performing these
measurements on a large scale is one of the new applications of the window
opened by current low frequency arrays. Using various simple axisymmetric
emissivity models and a number of GMF-based emissivity models we can simulate
the synchrotron emissivities and compare them to the observed values in the
catalog. We present a catalog of low-frequency absorption measurements of HII
regions, their distances and electron temperatures, compiled from literature.
These data show that the axisymmetric emissivity models are not complex enough,
but the GMF-based emissivity models deliver a reasonable fit. These models
suggest that the fit can be improved by either an enhanced synchrotron
emissivity in the outer reaches of the Milky Way, or an emissivity drop near
the Galactic center. State-of-the-art GMF models plus a constant CR density
model cannot explain low-frequency absorption measurements, but the fits
improved with slight (ad-hoc) adaptations. It is clear that more detailed
models are needed, but the current results are very promising.Comment: 14 pages, 9 figures, accepted for publication in A&
Structure in the polarized Galactic synchrotron emission, in particular `depolarization canals'
The polarized component of the diffuse radio synchrotron emission of our
Galaxy shows structure, which is apparently unrelated to the structure in total
intensity, on many scales. The structure in the polarized emission can be due
to several processes or mechanisms. Some of those are related to the
observational setup, such as beam depolarization -- the vector combination and
(partial) cancellation of polarization vectors within a synthesized beam --, or
the insensitivity of a synthesis telescope to structure on large scales, also
known as the 'missing short spacings problem'. Other causes for structure in
the polarization maps are intrinsic to the radiative transfer of the emission
in the warm ISM, which induces Faraday rotation and depolarization.
We use data obtained with the Westerbork Synthesis Radio Telescope at 5
frequencies near 350 MHz to estimate the importance of the various mechanisms
in producing structure in the linearly polarized emission. In the two regions
studied here, which are both at positive latitudes in the second Galactic
quadrant, the effect of 'missing short spacings' is not important. The
properties of the narrow depolarization 'canals' that are observed in abundance
lead us to conclude that they are mostly due to beam depolarization, and that
they separate regions with different rotation measures. As beam depolarization
only creates structure on the scale of the synthesized beam, most of the
structure on larger scales must be due to depth depolarization. We do not
discuss that aspect of the observations here, but in a companion paper we
derive information about the properties of the ISM from the structure of the
polarized emission.Comment: 12 pages, 10 figures, accepted for publication by A&
The Southern Galactic Plane Survey: Polarized Radio Continuum Observations and Analysis
The Southern Galactic Plane Survey (SGPS) is a radio survey in the 21 cm H I
line and in 1.4 GHz full-polarization continuum, observed with the Australia
Telescope Compact Array and the Parkes 64m single dish telescope. The survey
spans a Galactic longitude of 253 deg < l < 358 deg and a latitude of |b| < 1
deg at a resolution of 100 arcsec and a sensitivity below 1 mJy/beam. This
paper presents interferometer only polarized continuum survey data and
describes the data taking, analysis processes and data products. The primary
data products are the four Stokes parameters I, Q, U, and V in 25 overlapping
fields of 5.5 deg by 2 deg, from which polarized intensity, polarization angle
and rotation measure are calculated. We describe the effects of missing short
spacings, and discuss the importance of the polarized continuum data in the
SGPS for studies of fluctuations and turbulence in the ionized interstellar
medium and for studying the strength and structure of the Galactic magnetic
field.Comment: 15 pages, 3 figures, to appear in ApJ Supplement Series. Full
resolution version available at
http://astro.berkeley.edu/~marijke/sgps_polarization.pd
Structure in the Rotation Measure Sky
An analysis of structure in rotation measure (RM) across the sky based on the
RM catalog of Taylor et al. (2009) is presented. Several resolved RM structures
are identified with structure in the local ISM, including radio loops I, II,
and III, the Gum nebula, and the Orion-Eridanus super bubble. Structure
functions (SFs) of RM are presented for selected areas, and maps of SF
amplitude and slope across the sky are compared with H-alpha intensity and
diffuse polarized intensity. RM variance on an angular scale of 1 degree is
correlated with length of the line of sight through the Galaxy, with a
contribution from local structures. The slope of the SFs is less concentrated
to the Galactic plane and less correlated with length of the line of sight
through the Galaxy, suggesting a more local origin for RM structure on angular
scales ~ 10 degrees. The RM variance is a factor ~2 higher towards the SGP than
towards the NGP, reflecting a more wide-spread asymmetry between the northern
and southern Galactic hemispheres. Depolarization of diffuse Galactic
synchrotron emission at latitudes < 30 degrees can be explained largely by
Faraday dispersion related to small-scale variance in RM, but the errors allow
a significant contribution from differential Faraday rotation along the line of
sight.Comment: 13 pages, 14 figures. Figures 1-4 and 9-10 available as separate
files. Accepted for publication in the Astrophysical Journal A complete pdf
with all figures included is available at
http://www.ras.ucalgary.ca/~stil/Stil_2010_RM_structure.pd
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