85 research outputs found
Observed Faraday Effects in Damped Lyman-Alpha Absorbers and Lyman Limit Systems: The Magnetised Environment of Galactic Building Blocks at Redshift=2
Protogalactic environments are typically identified using quasar absorption
lines, and these galactic building blocks can manifest as Damped Lyman-Alpha
Absorbers (DLAs) and Lyman Limit Systems (LLSs). We use radio observations of
Faraday effects to test whether DLAs and LLSs host a magnetised medium, by
combining DLA and LLS detections throughout the literature with 1.4 GHz
polarization data from the NRAO VLA Sky Survey (NVSS). We obtain a control, a
DLA, and a LLS sample consisting of 114, 19, and 27 lines-of-sight respectively
- all of which are polarized at to ensure Rician bias is
negligible. Using a Bayesian framework, we are unable to detect either coherent
or random magnetic fields in DLAs: the regular coherent magnetic fields within
the DLAs must be G, and the lack of depolarization is consistent
with the weakly magnetised gas in DLAs being non-turbulent and quiescent.
However, we find mild suggestive evidence that LLSs have coherent magnetic
fields: after controlling for the redshift-distribution of our data, we find a
71.5% probability that LLSs have a higher RM than a control sample. We also
find strong evidence that LLSs host random magnetic fields, with a 95.5%
probability that LLS lines-of-sight have lower polarized fractions than a
control sample. The regular coherent magnetic fields within the LLSs must be
G, and the magnetised gas must be highly turbulent with a typical
scale on the order of -20 pc, which is similar to that of the Milky
Way. This is consistent with the standard dynamo pedagogy, whereby magnetic
fields in protogalaxies increase in coherence and strength as a function of
cosmic time. Our results are consistent with a hierarchical galaxy formation
scenario, with the DLAs, LLSs, and strong magnesium II (MgII) systems exploring
three different stages of magnetic field evolution in galaxies.Comment: Submitted to Ap
Broadband, radio spectro-polarimetric study of 100 radiative-mode and jet-mode AGN
We present the results from a broadband (1 to 3 GHz), spectro-polarimetry
study of the integrated emission from 100 extragalactic radio sources with the
ATCA, selected to be highly linearly polarized at 1.4 GHz. We use a general
purpose, polarization model-fitting procedure that describes the Faraday
rotation measure (RM) and intrinsic polarization structure of up to three
distinct polarized emission regions or 'RM components' of a source. Overall,
37%/52%/11% of sources are best fit by one/two/three RM components. However,
these fractions are dependent on the signal-to-noise ratio (S/N) in
polarization (more RM components more likely at higher S/N). In general, our
analysis shows that sources with high integrated degrees of polarization at 1.4
GHz have low Faraday depolarization, are typically dominated by a single RM
component, have a steep spectral index, and a high intrinsic degree of
polarization. After classifying our sample into radiative-mode and jet-mode
AGN, we find no significant difference between the Faraday rotation or Faraday
depolarization properties of jet-mode and radiative-mode AGN. However, there is
a statistically significant difference in the intrinsic degree of polarization
between the two types, with the jet-mode sources having more intrinsically
ordered magnetic field structures than the radiative-mode sources. We also find
a preferred perpendicular orientation of the intrinsic magnetic field structure
of jet-mode AGN with respect to the jet direction, while no clear preference is
found for the radiative-mode sources.Comment: 29 pages (including Appendix), 28 figures, 7 tables. Accepted for
publication in MNRA
Interacting Large-Scale Magnetic Fields and Ionised Gas in the W50/SS433 System
The W50/SS433 system is an unusual Galactic outflow-driven object of
debatable origin. We have used the Australia Telescope Compact Array (ATCA) to
observe a new 198 pointing mosaic, covering , and
present the highest-sensitivity full-Stokes data of W50 to date using
wide-field, wide-band imaging over a 2 GHz bandwidth centred at 2.1 GHz. We
also present a complementary H mosaic created using the Isaac Newton
Telescope Photometric H Survey of the Northern Galactic Plane (IPHAS).
The magnetic structure of W50 is found to be consistent with the prevailing
hypothesis that the nebula is a reanimated shell-like supernova remnant (SNR),
that has been re-energised by the jets from SS433. We observe strong
depolarization effects that correlate with diffuse H emission, likely
due to spatially-varying Faraday rotation measure (RM) fluctuations of
to 61 rad m on scales to 6 pc. We also report the discovery of
numerous, faint, H filaments that are unambiguously associated with the
central region of W50. These thin filaments are suggestive of a SNR's shock
emission, and almost all have a radio counterpart. Furthermore, an RM-gradient
is detected across the central region of W50, which we interpret as a loop
magnetic field with a symmetry axis offset by to the
east-west jet-alignment axis, and implying that the evolutionary processes of
both the jets and the SNR must be coupled. A separate RM-gradient is associated
with the termination shock in the Eastern ear, which we interpret as a
ring-like field located where the shock of the jet interacts with the
circumstellar medium. Future optical observations will be able to use the new
H filaments to probe the kinematics of the shell of W50, potentially
allowing for a definitive experiment on W50's formation history.Comment: Submitted to MNRA
Polarized point sources in the LOFAR Two-meter Sky Survey: A preliminary catalog
The polarization properties of radio sources at very low frequencies (h45m–15h30m right ascension, 45°–57° declination, 570 square degrees). We have produced a catalog of 92 polarized radio sources at 150 MHz at 4.′3 resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized sources at such low frequencies. We estimate a lower limit to the polarized source surface density at 150 MHz, with our resolution and sensitivity, of 1 source per 6.2 square degrees. We find that our Faraday depth measurements are in agreement with previous measurements and have significantly smaller errors. Most of our sources show significant depolarization compared to 1.4 GHz, but there is a small population of sources with low depolarization indicating that their polarized emission is highly localized in Faraday depth. We predict that an extension of this work to the full LOTSS data would detect at least 3400 polarized sources using the same methods, and probably considerably more with improved data processing
Radio polarization properties of quasars and active galaxies at high redshifts
We present the largest ever sample of radio polarization properties for z > 4 sources, with 14 sources having significant polarization detections. Using wide-band data from the Karl G. Jansky Very Large Array, we obtained the rest-frame total intensity and polarization properties of 37 radio sources, nine of which have spectroscopic redshifts in the range 1 ≤ z ≤ 1.4, with the other 28 having spectroscopic redshifts in the range 3.5 ≤ z ≤ 6.21. Fits are performed for the Stokes I and fractional polarization spectra, and Faraday rotation measures are derived using rotation measure synthesis and QU fitting. Using archival data of 476 polarized sources, we compare high-redshift (z > 3) source properties to a 15 GHz rest-frame luminosity matched sample of low-redshift (z -2 for z > 3 sources and 57 ± 4 rad m-2 for z < 3. Although there is some indication of lower intrinsic rotation measures at high-z possibly due to higher depolarization from the high-density environments, using several statistical tests we detect no significant difference between low- and high-redshift sources. Larger samples are necessary to determine any true physical difference
MAGNETIC FIELD DISORDER AND FARADAY EFFECTS ON THE POLARIZATION OF EXTRAGALACTIC RADIO SOURCES
LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies
Accepted for publication in Astronomy and AstrophysicsAims. The shape of low-frequency radio continuum spectra of normal galaxies is not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFAR Multifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large sample of nearby star-forming galaxies. Methods. Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. The spectra are explained through the use of a three-dimensional model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Results. Our galaxies' spectra are generally flatter at lower compared to higher frequencies: the median spectral index α low measured between ≈ 50 MHz and 1.5 GHz is -0.57 ± 0.01 while the high-frequency one α high, calculated between 1.3 GHz and 5 GHz, is -0.77 ± 0.03. As there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we argue that the observed flattening is not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990, ApJ, 352, 30). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flattening due to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxies of our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100-200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions. Our modelling of galaxy spectra suggests that the weak spectral flattening observed in the nearby galaxies studied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way's foreground on the spectra of all external galaxies is also expected at very low frequencies.Peer reviewedFinal Accepted Versio
The extraordinary linear polarisation structure of the southern Centaurus A lobe revealed by ASKAP
We present observations of linear polarisation in the southern radio lobe of
Centaurus A, conducted during commissioning of the Australian Square Kilometre
Array Pathfinder (ASKAP) telescope. We used 16 antennas to observe a 30 square
degree region in a single 12 hour pointing over a 240 MHz band centred on 913
MHz. Our observations achieve an angular resolution of arcseconds
(480 parsecs), a maximum recoverable angular scale of 30 arcminutes, and a
full-band sensitivity of 85 \muupJy beam. The resulting maps of
polarisation and Faraday rotation are amongst the most detailed ever made for
radio lobes, with of order 10 resolution elements covering the source. We
describe several as-yet unreported observational features of the lobe,
including its detailed peak Faraday depth structure, and intricate networks of
depolarised filaments. These results demonstrate the exciting capabilities of
ASKAP for widefield radio polarimetry.Comment: 10 pages, 6 figures. Accepted in "The Power of Faraday Tomography"
special issue of Galaxie
Diffuse polarized emission in the LOFAR Two-meter Sky Survey
Faraday tomography allows us to map diffuse polarized synchrotron emission
from our Galaxy and use it to interpret the magnetic field in the interstellar
medium (ISM). We have applied Faraday tomography to 60 observations from the
LOFAR Two-meter Sky Survey (LOTSS) and produced a Faraday depth cube mosaic
covering 568 square degrees at high Galactic latitudes, at 4.3' angular
resolution and 1 rad m Faraday depth resolution, with a typical noise
level of 50--100 Jy per point spread function (PSF) per rotation measure
spread function (RMSF) (40-80 mK RMSF). While parts of the images are
strongly affected by instrumental polarization, we observe diffuse polarized
emission throughout most of the field, with typical brightness between 1 and 6
K RMSF, and Faraday depths between and +25 rad m.
We observed many new polarization features, some up to 15 degrees in length.
These include two regions with very uniformly structured, linear gradients in
the Faraday depth; we measured the steepness of these gradients as 2.6 and 13
rad m deg. We also observed a relationship between one of the
gradients and an HI filament in the local ISM. Other ISM tracers were also
checked for correlations with our polarization data and none were found, but
very little signal was seen in most tracers in this region. We conclude that
the LOTSS data are very well suited for Faraday tomography, and that a
full-scale survey with all the LOTSS data has the potential to reveal many new
Galactic polarization features and map out diffuse Faraday depth structure
across the entire northern hemisphere.Comment: 23 pages, 16 figures, accepted by A&A. Figures have been degraded to
comply with arXiv file size limits; full resolution figures will be available
in the A&A versio
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