Faraday rotation occurs along every line of sight in the Galaxy; rotation measure (RM) synthesis allows a 3D representation of the interstellar magnetic field. This study uses data from the Global ...Magneto-Ionic Medium Survey, a combination of single-antenna spectro-polarimetric studies, including northern sky data from the Dominion Radio Astrophysical Observatory (DRAO) 26 m telescope (1270-1750 MHz) and southern sky data from the Parkes 64 m telescope (300-480 MHz). From the synthesized Faraday spectral cubes we compute the zeroth, first, and second moments to find the total polarized emission, mean RM, and RM width of the polarized emission. From DRAO first moments we find a weak vertical field directed from Galactic North to South, but Parkes data reveal fields directed toward the Sun at high latitudes in both hemispheres: the two surveys clearly sample different volumes. DRAO second moments show feature widths in Faraday spectra increasing with decreasing positive latitudes, implying that longer lines of sight encounter more Faraday rotating medium, but this is not seen at negative latitudes. Parkes data show the opposite: at positive latitudes the second moment decreases with decreasing latitude, but not at negative latitudes. Comparing first moments with RMs of pulsars and extragalactic sources and a study of depolarization together confirm that the DRAO survey samples to larger distances than the Parkes data. Emission regions in the DRAO survey are typically 700-1000 pc away, slightly beyond the scale height of the magneto-ionic medium; emission detected in the Parkes survey is entirely within the magneto-ionic disk, less than 500 pc away.
Abstract Angular dispersion functions are typically used to estimate the fluctuations in polarization angle around the mean magnetic field orientation in dense regions, such as molecular clouds. The ...technique provides accurate turbulent-to-regular magnetic field ratios, 〈 B t 2 〉 1 / 2 / B pos , which are often underestimated by the classic Davis–Chandrasekhar–Fermi method. We assess the technique's suitability to characterize the turbulent and regular plane-of-sky magnetic field in low-density structures of the nearby interstellar medium (ISM), particularly when the turbulence outer scale, δ , is smaller than the smallest scale observed, ℓ min . We use optical polarization maps of three intermediate-latitude fields (∣ b ∣ ≳ 7.°5) with dimensions of 0.°3 × 0.°3, sourced from the Interstellar Polarization Survey–General ISM catalog. We decomposed the H i emission detected by the Galactic All-Sky Survey within our fields to estimate the multiphase ISM properties associated with the structure coupled to the magnetic field. We produced maps of the plane-of-sky magnetic field strength ( B pos ), mass density ( ρ ), and turbulent velocity dispersion ( σ v ,turb ). In the regions with well-defined structures at d < 400 pc, the average B – ranges from ∼3 μ G to ∼9 μ G , depending on the method and physical properties. In the region where structures extend up to 1000 pc, B pos varies from ∼1 to ∼3 μ G . The results agree with previous estimations in the local, diffuse ISM. Finally, optical starlight polarization and thermal dust polarization at 353 GHz consistently reveal a highly regular plane-of-sky magnetic field orientation unfazed by diffuse dust structures observed at 12 μ m.
Abstract
Optical starlight can be partially polarized while propagating through the dusty, magnetized interstellar medium (ISM). The polarization efficiency describes the polarization intensity ...fraction per reddening unit,
P
V
/
E
(
B
−
V
), related to the interstellar dust grains and magnetic field properties. The maximum value observed,
P
V
/
E
(
B
−
V
)
max
, is thus achieved under optimal polarizing conditions of the ISM. Therefore, the analysis of polarization efficiency observations across the Galaxy contributes to the study of magnetic field topology, small-scale magnetic fluctuations, grain-alignment efficiency, and composition. Infrared observations from Planck satellite have set
P
V
/
E
(
B
−
V
)
max
to 13% mag
−1
. However, recent optical polarization observations in Planck's highly polarized regions showed polarization efficiency values between 13.6% mag
−1
and 18.2% mag
−1
(depending on the extinction map used), indicating that
P
V
/
E
(
B
−
V
)
max
is not well constrained yet. We used
V
-band polarimetry of the Interstellar Polarization Survey (consisting of ∼10,500 high-quality observations distributed in 34 fields of 0.°3 × 0.°3) to accurately estimate the polarization efficiency in the ISM. We estimated the upper limit of
P
V
/
E
(
B
−
V
) with the weighted 99th percentile of the field. In five regions, the polarization efficiency upper limit is above 13% mag
−1
. Furthermore, we found
P
V
/
E
(
B
−
V
)
max
=
15.8
−
0.9
+
1.3
% mag
−1
using diffuse intermediate-latitude (∣
b
∣ > 7.°5) regions with apparently strong regular Galactic magnetic field in the plane-of-sky. We studied the variations of
P
V
/
E
(
B
−
V
) across the sky and tested toy models of polarization efficiency with Galactic longitude that showed some correspondence with a uniform spiral magnetic field.
Abstract Starlight polarimetry, when combined with accurate distance measurements, allows for exploration of the three-dimensional structure of local magnetic fields in great detail. We present ...optical polarimetric observations of stars in and close to the Southern Coalsack, taken from the Interstellar Polarization Survey. Located in five fields of view approximately 0.°3 × 0.°3 in size, these data represent the highest density of optical polarimetric observations in the Southern Coalsack to date. Using these data, combined with accurate distances and extinctions based on Gaia data, we are able to characterize the magnetic field of the Coalsack and disentangle contributions to the polarization caused by the Southern Coalsack and a background structure. For the Southern Coalsack, we find an average magnetic field orientation of θ ∼ 75° with respect to the Galactic north pole and an average plane-of-sky magnetic field strength of approximately B POS = 10 μ G, using the Davis–Chandrasekhar–Fermi method. These values are in agreement with some earlier estimates of the Coalsack’s magnetic field. In order to study the distant structure, we introduce a simple method to separate and isolate the polarization of distant stars from foreground contribution. For the distant structure, which we estimate to be located at a distance of approximately 1.3–1.5 kpc, we find an average magnetic field orientation of θ ∼ 100° and estimate a field strength of B POS ∼ 10 μ G, although this will remain highly uncertain until the precise nature of the distant structure can be uncovered.
The grand-design face-on spiral galaxy M 51 is an excellent laboratory for studying magnetic fields in galaxies. Due to wavelength-dependent Faraday depolarization, linearly polarized synchrotron ...emission at different radio frequencies yields a picture of the galaxy at different depths: observations in the
L
-band (1–2 GHz) probe the halo region, while at 4.85 GHz (
C
-band) and 8.35 GHz (
X
-band), the linearly polarized emission mostly emerges from the disk region of M 51. We present new observations of M 51 using the
Karl G. Jansky
Very Large Array at the intermediate frequency range of the
S
-band (2–4 GHz), where previously no high-resolution broadband polarization observations existed, to shed new light on the transition region between the disk and the halo. We present the
S
-band radio images of the distributions of the total intensity, polarized intensity, degree of polarization, and rotation measure (RM). The RM distribution in the
S
-band shows a fluctuating pattern without any apparent large-scale structure. We discuss a model of the depolarization of synchrotron radiation in a multi-layer magneto-ionic medium and compare the model predictions to the multi-frequency polarization data of M 51 between 1–8 GHz. The model makes distinct predictions of a two-layer (disk–halo) and three-layer (far-side halo “disk” near-side halo) system. Since the model predictions strongly differ within the wavelength range of the
S
-band, the new
S
-band data are essential for distinguishing between the different systems. A two-layer model of M 51 is preferred. The parameters of the model are adjusted to fit to the data of polarization fractions in a few selected regions. In three spiral arm regions, the turbulent field in the disk dominates with strengths between 18
μ
G and 24
μ
G, while the regular field strengths are 8 − 16
μ
G. In one inter-arm region, the regular field strength of 18
μ
G exceeds that of the turbulent field of 11
μ
G. The regular field strengths in the halo are 3 − 5
μ
G. The observed RMs in the disk-halo transition region are probably dominated by tangled regular fields, as predicted from models of evolving dynamos, and/or vertical fields, as predicted from numerical simulations of Parker instabilities or galactic winds. Both types of magnetic fields have frequent reversals on scales similar to or larger than the beam size (∼550 pc) that contribute to an increase of the RM dispersion and to distortions of any large-scale pattern of the regular field. Our study devises new ways of analyzing and interpreting broadband multi-frequency polarization data that will be applicable to future data from, for example, the Square Kilometre Array.
Abstract
We present the S-Band Polarization All Sky Survey (S-PASS), a survey of polarized radio emission over the southern sky at Dec. <−1° taken with the Parkes radio telescope at 2.3 GHz. The main ...aim was to observe at a frequency high enough to avoid strong depolarization at intermediate Galactic latitudes (still present at 1.4 GHz) to study Galactic magnetism, but low enough to retain ample signal-to-noise ratio (S/N) at high latitudes for extragalactic and cosmological science. We developed a new scanning strategy based on long azimuth scans and a corresponding map-making procedure to make recovery of the overall mean signal of Stokes Q and U possible, a long-standing problem with polarization observations. We describe the scanning strategy, map-making procedure and validation tests. The overall mean signal is recovered with a precision better than 0.5 per cent. The maps have a mean sensitivity of 0.81 mK on beam-size scales and show clear polarized signals, typically to within a few degrees of the Galactic plane, with ample S/N everywhere (the typical signal in low-emission regions is 13 mK and 98.6 per cent of pixels have S/N > 3). The largest depolarization areas are in the inner Galaxy, associated with the Sagittarius Arm. We have also computed a rotation measure map combining S-PASS with archival data from the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck experiments. A Stokes I map has been generated, with sensitivity limited to the confusion level of 9 mK.
Abstract
Magnetic fields permeate the entire Galaxy and are essential to, for example, the regulation of several stages of the star formation process and cosmic-ray transportation. Unraveling their ...properties, such as intensity and topology, is an observational challenge that requires combining different and complementary techniques. The polarization of starlight due to the absorption by field-aligned nonspherical dust grains provides a unique source of information about the interstellar magnetic field in the optical band. This work introduces a first analysis of a new catalog of optical observations of linearly polarized starlight in the diffuse interstellar medium (ISM), the Interstellar Polarization Survey–General ISM (IPS-GI). We used data from the IPS-GI, focusing on 38 fields sampling lines of sight in the diffuse medium. The fields are about 0.°3 × 0.°3 in size, and each of them contains ∼1000 stars, on average. The IPS-GI catalog has polarimetric measurements of over 40,000 stars, over 18,000 of which have
P
/
σ
P
> 5. We added distances and other parameters from auxiliary catalogs to over 36,000 of these stars. We analyzed parameter distributions and correlations between parameters of a high-quality subsample of 10,516 stars (i.e., ∼275 stars per field). As expected, the degree of polarization tends to increase with the extinction, producing higher values of polarization at greater distances or lower absolute Galactic latitudes. Furthermore, we find evidence for a large-scale ordered Galactic magnetic field.
Context.
Cosmic rays (CRs) and the Galactic magnetic field (GMF) are fundamental actors in many processes in the Milky Way. The observed interaction product of these actors is Galactic synchrotron ...emission integrated over the line of sight (LOS). A comparison to simulations can be made with this tracer using existing GMF models and CR density models. This probes the GMF strength and morphology and the CR density.
Aims.
Our aim is to provide insight into the Galactic CR density and the distribution and morphology of the GMF strength by exploring and explaining the differences between the simulations and observations of synchrotron intensity.
Methods.
At low radio frequencies HII regions become opaque due to free–free absorption. Using these HII regions we can measure the synchrotron intensity over a part of the LOS through the Galaxy. The measured intensity per unit path length, that is, the emissivity, for HII regions at different distances, allows us to probe the variation in synchrotron emission not only across the sky but also in the third dimension of distance. Performing these measurements on a large scale is one of the new applications of the window opened by current low-frequency arrays. Using a number of existing GMF models in conjunction with the Galactic CR modeling code GALPROP, we can simulate these synchrotron emissivities.
Results.
We present an updated catalog, compiled from the literature, of low-frequency absorption measurements of HII regions, their distances, and electron temperatures. We report a simulated emissivity that shows a compatible trend for HII regions that are near the observer. However, we observe a systematically increasing synchrotron emissivity for HII regions that are far from the observer, which is not compatible with the values simulated by the GMF models and GALPROP.
Conclusions.
Current GMF models plus a GALPROP generated CR density model cannot explain low-frequency absorption measurements. One possibility is that distances to all HII regions catalogued at the kinematic “far” distance are erroneously determined, although this is unlikely since it ignores all evidence for far distances in the literature. However, a detection bias due to the nature of this tracer requires us to keep in mind that certain sources may be missed in an observation. The other possibilities are an enhanced emissivity in the outer Galaxy or a diminished emissivity in the inner Galaxy.
The Galactic Faraday rotation sky 2020 Hutschenreuter, S.; Anderson, C. S.; Betti, S. ...
Astronomy and astrophysics (Berlin),
01/2022, Volume:
657
Journal Article
Peer reviewed
Open access
Aims.
This work provides an update to existing reconstructions of the Galactic Faraday rotation sky by processing almost all Faraday rotation data sets available at the end of the year 2020. ...Observations of extra-Galactic sources in recent years have further illuminated the previously underconstrained southern celestial sky, as well as parts of the inner disc of the Milky Way, along with other regions. This has culminated in an all-sky data set of 55 190 data points, thereby comprising a significant expansion on the 41 330 used in previous works. At the same time, this novelty makes an updated separation of the Galactic component a promising enterprise. The increased source density allows us to present our results in a resolution of about 1.3 × 10
−2
deg
2
(46.8 arcmin
2
), which is a twofold increase compared to previous works.
Methods.
As for previous Faraday rotation sky reconstructions, this work is based on information field theory, namely, a Bayesian inference scheme for field-like quantities that handles noisy and incomplete data.
Results.
In contrast to previous reconstructions, we find a significantly thinner and pronounced Galactic disc with small-scale structures exceeding values of several thousand rad m
−2
. The improvements can mainly be attributed to the new catalog of Faraday data, but are also supported by advances in correlation structure modeling within numerical information field theory. We also provide a detailed discussion on the statistical properties of the Faraday rotation sky and we investigate correlations with other data sets.
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