84 research outputs found
A new method to suppress the bias in polarized intensity
Computing polarised intensities from noisy data in Stokes U and Q suffers
from a positive bias that should be suppressed. To develop a correction method
that, when applied to maps, should provide a distribution of polarised
intensity that closely follows the signal from the source. We propose a new
method to suppress the bias by estimating the polarisation angle of the source
signal in a noisy environment with help of a modified median filter. We then
determine the polarised intensity, including the noise, by projection of the
observed values of Stokes U and Q onto the direction of this polarisation
angle. We show that our new method represents the true signal very well. If the
noise distribution in the maps of U and Q is Gaussian, then in the corrected
map of polarised intensity it is also Gaussian. Smoothing to larger Gaussian
beamsizes, to improve the signal-to-noise ratio, can be done directly with our
method in the map of the polarised intensity. Our method also works in case of
non-Gaussian noise distributions. The maps of the corrected polarised
intensities and polarisation angles are reliable even in regions with weak
signals and provide integrated flux densities and degrees of polarisation
without the cumulative effect of the bias, which especially affects faint
sources. Features at low intensity levels like 'depolarisation canals' are
smoother than in the maps using the previous methods, which has broader
implications, for example on the interpretation of interstellar turbulence.Comment: accepted for publication in Astronomy & Astrophysic
The NOD3 software package: A graphical user interface-supported reduction package for single-dish radio continuum and polarisation observations
The venerable NOD2 data reduction software package for single-dish radio
continuum observations, developed for use at the 100-m Effelsberg radio
telescope, has been successfully applied over many decades. Modern computing
facilities call for a new design.
We aim to develop an interactive software tool with a graphical user
interface (GUI) for the reduction of single-dish radio continuum maps. Special
effort is given on the reduction of distortions along the scanning direction
(scanning effects) by combining maps scanned in orthogonal directions or dual-
or multiple-horn observations that need to be processed in a restoration
procedure. The package should also process polarisation data and offer the
possibility to include special tasks written by the individual user.
Based on the ideas of the NOD2 package we developed NOD3, which includes all
necessary tasks from the raw maps to the final maps in total intensity and
linear polarisation. Furthermore, plot routines and several methods for map
analysis are available. The NOD3 package is written in Python which allows to
extend the package by additional tasks. The required data format for the input
maps is FITS.
NOD3 is a sophisticated tool to process and analyse maps from single-dish
observations that are affected by 'scanning effects' due to clouds, receiver
instabilities, or radio-frequency interference (RFI). The 'basket-weaving' tool
combines orthogonally scanned maps to a final map that is almost free of
scanning effects. The new restoration tool for dual-beam observations reduces
the noise by a factor of about two compared to the NOD2 version. Combining
single-dish with interferometer data in the map plane ensures the full recovery
of the total flux density.Comment: 8 pages, 10 figures, Accepted for publication in A&
Enhancement of magnetic fields arising from galactic encounters
Galactic encounters are usually marked by a substantial increase of
synchrotron emission of the interacting galaxies compared to the typical
emission from similar isolated galaxies. This is believed to be associated with
an increase of the star formation rate and the associated turbulent magnetic
fields. The regular magnetic field is usually believed to decrease. We consider
a simple, however rather realistic, mean-field galactic dynamo model where the
effects of small-scale generation are represented by random injections of
magnetic field from star forming regions. We represent an encounter by the
introduction of large-scale streaming velocities and by an increase in
small-scale magnetic field injections. The latter describes the effect of an
increase of the star formation rate caused by the encounter. We demonstrate
that large-scale streaming, with associated deviations in the rotation curve,
can result in an enhancement of the anisotropic turbulent (ordered) magnetic
field strength, mainly along the azimuthal direction, leading to a significant
temporary increase of the total magnetic energy during the encounter; the
representation of an increase in star formation rate has an additional strong
effect. In contrast to expectations, the large-scale (regular) magnetic field
structure is not significantly destroyed by the encounter. It may be somewhat
weakened for a relatively short period, and its direction after the encounter
may be reversed. The encounter causes enhanced total and polarized emission
without increase of the regular magnetic field strength. The increase of
synchrotron emission caused by the large-scale streaming can be comparable to
the effect of the increase of the star formation rate, depending on the choice
of parameters.The effects of the encounter on the total magnetic field energy
last only slightly longer than the duration of the encounter (ca. 1 Gyr).Comment: 9 pages, 11 figures minor changes in response to referee's
comments+linguistic/stylistic change
The large scale magnetic field structure of the spiral galaxy NGC 5775
In order to better determine the large-scale 3D-structure of magnetic fields
in spiral galaxies we present a Faraday rotation analysis of the edge-on spiral
galaxy NGC 5775. Deep radio-continuum observations in total power and linear
polarization were performed at 8.46 GHz with the VLA and the 100-m Effelsberg
telescope. They were analyzed together with archival 4.86 and 1.49 GHz
VLA-data. We thus can derive rotation measures from a comparison of three
frequencies and determine the intrinsic magnetic field structure. A very
extended halo is detected in NGC 5775, with magnetic field lines forming an
X-shaped structure. Close to the galactic disk the magnetic field is
plane-parallel. The scaleheights of the radio emission esimated for NGC 5775
are comaprable with other galaxies. The rotation measure distribution varies
smoothly on both sides along the major axis from positive to negative values.
From the derived distribution of rotation measures and the plane-parallel
intrinsic magnetic field orientation along the galactic midplane we conclude
that NGC 5775 has an 'even axisymmetric' large-scale magnetic field
configuration in the disk as generated by an \alpha \Omega -dynamo which is
accompanied by a quadrupolar poloidal field. The magnetic field lines of the
plane-parallel component are pointing 'outwards'. The observed X-shaped halo
magnetic field, however, cannot be explained by the action of the disk's
mean-field dynamo alone. It is probably due to the influence of the galactic
wind together with the dynamo action.Comment: 11 pages, 13 figures, A&A accepte
Lack of Interaction between the Dust Grains and the Anomalous Radio Jet in the Nearby Spiral Galaxy NGC 4258
We obtained Spitzer/IRAC 3.6-8 micron images of the nearby spiral galaxy NGC
4258 to study possible interactions between dust and the radio jet. In our
analysis we also included high-resolution radio continuum, H-alpha, CO, and
X-ray data. Our data reveal that the 8 micron emission, believed to originate
largely from PAH molecules and hot dust, is an excellent tracer of the normal
spiral structure in NGC 4258, and hence it originates from the galactic plane.
We investigated the possibility of dust destruction by the radio jet by
calculating correlation coefficients between the 8 micron and radio continuum
emissions along the jet in two independent ways, namely (i) from
wavelet-transformed maps of the original images at different spatial scales,
and (ii) from one-dimensional intensity cuts perpendicular to the projected
path of the radio jet on the sky. No definitive sign of a correlation (or
anticorrelation) was detected on relevant spatial scales with either approach,
implying that any dust destruction must take place at spatial scales that are
not resolved by our observations.Comment: Accepted for publication in the Astronomical Journal (11 pages, 10
figures, 1 table
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