| 1. |
- Adam, R., et al.
(author)
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Planck intermediate results XLII. Large-scale Galactic magnetic fields
- 2016
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 596
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Journal article (peer-reviewed)abstract
- Recent models for the large-scale Galactic magnetic fields in the literature have been largely constrained by synchrotron emission and Faraday rotation measures. We use three different but representative models to compare their predicted polarized synchrotron and dust emission with that measured by the Planck satellite. We first update these models to match the Planck synchrotron products using a common model for the cosmic-ray leptons. We discuss the impact on this analysis of the ongoing problems of component separation in the Planck microwave bands and of the uncertain cosmic-ray spectrum. In particular, the inferred degree of ordering in the magnetic fields is sensitive to these systematic uncertainties, and we further show the importance of considering the expected variations in the observables in addition to their mean morphology. We then compare the resulting simulated emission to the observed dust polarization and find that the dust predictions do not match the morphology in the Planck data but underpredict the dust polarization away from the plane. We modify one of the models to roughly match both observables at high latitudes by increasing the field ordering in the thin disc near the observer. Though this specific analysis is dependent on the component separation issues, we present the improved model as a proof of concept for how these studies can be advanced in future using complementary information from ongoing and planned observational projects.
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| 2. |
- Aghanim, N., et al.
(author)
-
Planck 2018 results XII. Galactic astrophysics using polarized dust emission
- 2020
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 641
-
Journal article (peer-reviewed)abstract
- Observations of the submillimetre emission from Galactic dust, in both total intensity I and polarization, have received tremendous interest thanks to the Planck full-sky maps. In this paper we make use of such full-sky maps of dust polarized emission produced from the third public release of Planck data. As the basis for expanding on astrophysical studies of the polarized thermal emission from Galactic dust, we present full-sky maps of the dust polarization fraction p, polarization angle psi, and dispersion function of polarization angles ?. The joint distribution (one-point statistics) of p and N-H confirms that the mean and maximum polarization fractions decrease with increasing N-H. The uncertainty on the maximum observed polarization fraction, (max) = 22.0(-1.4)(+3.5) p max = 22 . 0 - 1.4 + 3.5 % at 353 GHz and 80 ' resolution, is dominated by the uncertainty on the Galactic emission zero level in total intensity, in particular towards diffuse lines of sight at high Galactic latitudes. Furthermore, the inverse behaviour between p and ? found earlier is seen to be present at high latitudes. This follows the ?proportional to p(-1) relationship expected from models of the polarized sky (including numerical simulations of magnetohydrodynamical turbulence) that include effects from only the topology of the turbulent magnetic field, but otherwise have uniform alignment and dust properties. Thus, the statistical properties of p, psi, and ? for the most part reflect the structure of the Galactic magnetic field. Nevertheless, we search for potential signatures of varying grain alignment and dust properties. First, we analyse the product map ?xp, looking for residual trends. While the polarization fraction p decreases by a factor of 3-4 between N-H=10(20) cm(-2) and N-H=2x10(22) cm(-2), out of the Galactic plane, this product ?xp only decreases by about 25%. Because ? is independent of the grain alignment efficiency, this demonstrates that the systematic decrease in p with N-H is determined mostly by the magnetic-field structure and not by a drop in grain alignment. This systematic trend is observed both in the diffuse interstellar medium (ISM) and in molecular clouds of the Gould Belt. Second, we look for a dependence of polarization properties on the dust temperature, as we would expect from the radiative alignment torque (RAT) theory. We find no systematic trend of ?xp with the dust temperature T-d, whether in the diffuse ISM or in the molecular clouds of the Gould Belt. In the diffuse ISM, lines of sight with high polarization fraction p and low polarization angle dispersion ? tend, on the contrary, to have colder dust than lines of sight with low p and high ?. We also compare the Planck thermal dust polarization with starlight polarization data in the visible at high Galactic latitudes. The agreement in polarization angles is remarkable, and is consistent with what we expect from the noise and the observed dispersion of polarization angles in the visible on the scale of the Planck beam. The two polarization emission-to-extinction ratios, R-P/p and R-S/V, which primarily characterize dust optical properties, have only a weak dependence on the column density, and converge towards the values previously determined for translucent lines of sight. We also determine an upper limit for the polarization fraction in extinction, p(V)/E(B-V), of 13% at high Galactic latitude, compatible with the polarization fraction p approximate to 20% observed at 353 GHz. Taken together, these results provide strong constraints for models of Galactic dust in diffuse gas.
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| 3. |
- Aghanim, N., et al.
(author)
-
Planck intermediate results XLIV. Structure of the Galactic magnetic field from dust polarization maps of the southern Galactic cap
- 2016
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 596
-
Journal article (peer-reviewed)abstract
- Using data from the Planck satellite, we study the statistical properties of interstellar dust polarization at high Galactic latitudes around the south pole (b < -60 degrees). Our aim is to advance the understanding of the magnetized interstellar medium (ISM), and to provide a modelling framework of the polarized dust foreground for use in cosmic microwave background (CMB) component-separation procedures. We examine the Stokes I, Q, and U maps at 353 GHz, and particularly the statistical distribution of the polarization fraction (p) and angle (Psi), in order to characterize the ordered and turbulent components of the Galactic magnetic field (GMF) in the solar neighbourhood. The Q and U maps show patterns at large angular scales, which we relate to the mean orientation of the GMF towards Galactic coordinates (l(0); b(0)) = (70 degrees +/- 5 degrees, 24 degrees +/- 5 degrees). The histogram of the observed p values shows a wide dispersion up to 25%. The histogram Psi of has a standard deviation of 12 degrees about the regular pattern expected from the ordered GMF. We build a phenomenological model that connects the distributions of p and Psi to a statistical description of the turbulent component of the GMF, assuming a uniform effective polarization fraction (p(0)) of dust emission. To compute the Stokes parameters, we approximate the integration along the line of sight (LOS) as a sum over a set of N independent polarization layers, in each of which the turbulent component of the GMF is obtained from Gaussian realizations of a power-law power spectrum. We are able to reproduce the observed p and distributions using a p0 value of 26%, a ratio of 0.9 between the strengths of the turbulent and mean components of the GMF, and a small value of N. The mean value of p (inferred from the fit of the large-scale patterns in the Stokes maps) is 12 +/- 1%. We relate the polarization layers to the density structure and to the correlation length of the GMF along the LOS. We emphasize the simplicity of our model (involving only a few parameters), which can be easily computed on the celestial sphere to produce simulated maps of dust polarization. Our work is an important step towards a model that can be used to assess the accuracy of component-separation methods in present and future CMB experiments designed to search the B mode CMB polarization from primordial gravity waves.
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| 4. |
- Akrami, Y., et al.
(author)
-
Planck 2018 results : XI. Polarized dust foregrounds
- 2020
-
In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 641
-
Journal article (peer-reviewed)abstract
- The study of polarized dust emission has become entwined with the analysis of the cosmic microwave background (CMB) polarization in the quest for the curl-like B-mode polarization from primordial gravitational waves and the low-multipole E-mode polarization associated with the reionization of the Universe. We used the new Planck PR3 maps to characterize Galactic dust emission at high latitudes as a foreground to the CMB polarization and use end-to-end simulations to compute uncertainties and assess the statistical significance of our measurements. We present PlanckEE, BB, and TE power spectra of dust polarization at 353 GHz for a set of six nested high-Galactic-latitude sky regions covering from 24 to 71% of the sky. We present power-law fits to the angular power spectra, yielding evidence for statistically significant variations of the exponents over sky regions and a difference between the values for the EE and BB spectra, which for the largest sky region are alpha (EE)=-2.42 +/- 0.02 and alpha (BB)=-2.54 +/- 0.02, respectively. The spectra show that the TE correlation and E/B power asymmetry discovered by Planck extend to low multipoles that were not included in earlier Planck polarization papers due to residual data systematics. We also report evidence for a positive TB dust signal. Combining data from Planck and WMAP, we have determined the amplitudes and spectral energy distributions (SEDs) of polarized foregrounds, including the correlation between dust and synchrotron polarized emission, for the six sky regions as a function of multipole. This quantifies the challenge of the component-separation procedure that is required for measuring the low-l reionization CMB E-mode signal and detecting the reionization and recombination peaks of primordial CMB B modes. The SED of polarized dust emission is fit well by a single-temperature modified black-body emission law from 353 GHz to below 70 GHz. For a dust temperature of 19.6 K, the mean dust spectral index for dust polarization is beta (P)(d) = 1.53 +/- 0.02 beta d P = 1.53 +/- 0.02 . The difference between indices for polarization and total intensity is beta (P)(d)-beta (I)(d) = 0.05 +/- 0.03 beta d P - beta d I =0.05 +/- 0.03 . By fitting multi-frequency cross-spectra between Planck data at 100, 143, 217, and 353 GHz, we examine the correlation of the dust polarization maps across frequency. We find no evidence for a loss of correlation and provide lower limits to the correlation ratio that are tighter than values we derive from the correlation of the 217- and 353 GHz maps alone. If the Planck limit on decorrelation for the largest sky region applies to the smaller sky regions observed by sub-orbital experiments, then frequency decorrelation of dust polarization might not be a problem for CMB experiments aiming at a primordial B-mode detection limit on the tensor-to-scalar ratio r similar or equal to 0.01 at the recombination peak. However, the Planck sensitivity precludes identifying how difficult the component-separation problem will be for more ambitious experiments targeting lower limits on r.
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| 5. |
- André, Ph., et al.
(author)
-
Probing the cold magnetised Universe with SPICA-POL (B-BOP)
- 2019
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In: Publications Astronomical Society of Australia. - : Cambridge University Press (CUP). - 1323-3580 .- 1448-6083. ; 36
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Research review (peer-reviewed)abstract
- Space Infrared Telescope for Cosmology and Astrophysics (SPICA), the cryogenic infrared space telescope recently pre-selected for a 'Phase A' concept study as one of the three remaining candidates for European Space Agency (ESA's) fifth medium class (M5) mission, is foreseen to include a far-infrared polarimetric imager [SPICA-POL, now called B-fields with BOlometers and Polarizers (B-BOP)], which would offer a unique opportunity to resolve major issues in our understanding of the nearby, cold magnetised Universe. This paper presents an overview of the main science drivers for B-BOP, including high dynamic range polarimetric imaging of the cold interstellar medium (ISM) in both our Milky Way and nearby galaxies. Thanks to a cooled telescope, B-BOP will deliver wide-field 100-350 mu m images of linearly polarised dust emission in Stokes Q and U with a resolution, signal-to-noise ratio, and both intensity and spatial dynamic ranges comparable to those achieved by Herschel images of the cold ISM in total intensity (Stokes I). The B-BOP 200 mu m images will also have a factor similar to 30 higher resolution than Planck polarisation data. This will make B-BOP a unique tool for characterising the statistical properties of the magnetised ISM and probing the role of magnetic fields in the formation and evolution of the interstellar web of dusty molecular filaments giving birth to most stars in our Galaxy. B-BOP will also be a powerful instrument for studying the magnetism of nearby galaxies and testing Galactic dynamo models, constraining the physics of dust grain alignment, informing the problem of the interaction of cosmic rays with molecular clouds, tracing magnetic fields in the inner layers of protoplanetary disks, and monitoring accretion bursts in embedded protostars.
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| 6. |
- Van Eck, C.L., et al.
(author)
-
Polarized point sources in the LOFAR Two-meter Sky Survey: A preliminary catalog
- 2018
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 613
-
Research review (peer-reviewed)abstract
- The polarization properties of radio sources at very low frequencies (<200 MHz) have not been widely measured, but the new generation of low-frequency radio telescopes, including the Low Frequency Array (LOFAR: a Square Kilometre Array Low pathfinder), now gives us the opportunity to investigate these properties. In this paper, we report on the preliminary development of a data reduction pipeline to carry out polarization processing and Faraday tomography for data from the LOFAR Two-meter Sky Survey (LOTSS) and present the results of this pipeline from the LOTSS preliminary data release region (10h45m–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.
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| 7. |
- Beck, R., et al.
(author)
-
Structure, dynamical impact and origin of magnetic fields in nearby galaxies in the SKA era
- 2015
-
In: Proceedings of Science. - 1824-8039. ; 9-13-June-2014
-
Conference paper (peer-reviewed)abstract
- Magnetic fields are an important ingredient of the interstellar medium (ISM). Besides their importancefor star formation, they govern the transport of cosmic rays, relevant to the launch andregulation of galactic outflows and winds, which in turn are pivotal in shaping the structure of halomagnetic fields. Mapping the small-scale structure of interstellar magnetic fields in many nearbygalaxies is crucial to understand the interaction between gas and magnetic fields, in particularhow gas flows are affected. Elucidation of the magnetic role in, e.g., triggering star formation,forming and stabilising spiral arms, driving outflows, gas heating by reconnection and magnetisingthe intergalactic medium has the potential to revolutionise our physical picture of the ISMand galaxy evolution in general. Radio polarisation observations in the very nearest galaxies athigh frequencies (3 GHz) and with high spatial resolution (500) hold the key here. The galaxysurvey with SKA1 that we propose will also be a major step to understand the galactic dynamo,which is important for models of galaxy evolution and for astrophysical magnetohydrodynamicsin general. Field amplification by turbulent gas motions, which is crucial for efficient dynamoaction, has been investigated so far only in simulations, while compelling evidence of turbulentfields from observations is still lacking.
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| 8. |
- Jean, P., et al.
(author)
-
Positron transport in the interstellar medium
- 2009
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 508:3, s. 1099-1116
-
Journal article (peer-reviewed)abstract
- Aims. We seek to understand the propagation mechanisms of positrons in the interstellar medium (ISM). This understanding is a key to determine whether the spatial distribution of the annihilation emission observed in our Galaxy reflects the spatial distribution of positron sources and, therefore, makes it possible to place constraints on the origin of positrons. Methods. We review the different processes that are likely to affect the transport of positrons in the ISM. These processes fall into three broad categories: scattering off magnetohydrodynamic waves, collisions with particles of the interstellar gas, and advection with large-scale fluid motions. We assess the efficiency of each process and describe its impact on the propagation of positrons. We also develop a model of positron propagation, based on Monte-Carlo simulations, which enable us to estimate the distances traveled by positrons in the different phases of the ISM. Results. We find that low-energy (less than or similar to 10 MeV) positrons generally have negligible interactions with magnetohydrodynamic waves, insofar as these waves are heavily damped. Positron propagation is mainly controlled by collisions with gas particles. Under these circumstances, positrons can travel very large distances (up to similar to 30 kpc/n(H,cm-3) for 1 MeV positrons) along magnetic field lines before annihilating.
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| 9. |
- Van Eck, C.L., et al.
(author)
-
Faraday tomography of the local interstellar medium with LOFAR: Galactic foregrounds towards IC 342∗
- 2017
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 597, s. A98-
-
Journal article (peer-reviewed)abstract
- Magnetic fields pervade the interstellar medium (ISM), but are difficult to detect and characterize. The new generation of low-frequency radio telescopes, such as the Low Frequency Array (LOFAR: a Square Kilometre Array-low pathfinder), provides advancements in our capability of probing Galactic magnetism through low-frequency polarimetry. Maps of diffuse polarized radio emission and the associated Faraday rotation can be used to infer properties of, and trace structure in, the magnetic fields in the ISM. However, to date very little of the sky has been probed at high angular and Faraday depth resolution. We observed a 5° by 5° region centred on the nearby galaxy IC 342 (ℓ = 138.2°,b = + 10.6°) using the LOFAR high-band antennae in the frequency range 115-178 MHz. We imaged this region at 4′.5x3′.84.5 × 3.8 resolution and performed Faraday tomography to detect foreground Galactic polarized synchrotron emission separated by Faraday depth (different amounts of Faraday rotation). Our Faraday depth cube shows a rich polarized structure, with up to 30 K of polarized emission at 150 MHz. We clearly detect two polarized features that extend over most of the field, but are clearly separated in Faraday depth. Simulations of the behaviour of the depolarization of Faraday-thick structures at such low frequencies show that such structures would be too strongly depolarized to explain the observations. These structures are therefore rejected as the source of the observed polarized features. Only Faraday thin structures will not be strongly depolarized at low frequencies; producing such structures requires localized variations in the ratio of synchrotron emissivity to Faraday depth per unit distance. Such variations can arise from several physical phenomena, such as a transition between regions of ionized and (mostly) neutral gas. We conclude that the observed polarized emission is Faraday thin, and propose that the emission originates from two mostly neutral clouds in the local ISM. Using maps of the local ISM to estimate distances to these clouds, we have modelled the Faraday rotation for this line of sight and estimated that the strength of the line of sight component of magnetic field of the local ISM for this direction varies between-0.86 and+0.12 μG (where positive is towards the Earth). We propose that this may be a useful method for mapping magnetic fields within the local ISM in all directions towards nearby neutral clouds.
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