| 1. |
- Pelgrims, Vincent, et al.
(författare)
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Evidence for line-of-sight frequency decorrelation of polarized dust emission in Planck data
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 647
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Tidskriftsartikel (refereegranskat)abstract
- If a single line of sight (LOS) intercepts multiple dust clouds with different spectral energy distributions and magnetic field orientations, then the frequency scaling of each of the Stokes Q and U parameters of the thermal dust emission may be different, a phenomenon we refer to as LOS frequency decorrelation. We present first evidence for LOS frequency decorrelation in Planck data using independent measurements of neutral-hydrogen (HI) emission to probe the 3D structure of the magnetized interstellar medium (ISM). We use HI-based measurements of the number of clouds per LOS and the magnetic field orientation in each cloud to select two sets of sightlines: (i) a target sample of pixels that are likely to exhibit LOS frequency decorrelation and (ii) a control sample of pixels that lack complex LOS structure. We test the null hypothesis that LOS frequency decorrelation is not detectable in Planck 353 and 217 GHz polarization data at high Galactic latitudes. We reject the null hypothesis at high significance based on data that show that the combined effect of polarization angle variation with frequency and depolarization are detected in the target sample. This detection is robust against the choice of cosmic microwave background (CMB) map and map-making pipeline. The observed change in polarization angle due to LOS frequency decorrelation is detectable above the Planck noise level. The probability that the detected effect is due to noise alone ranges from 5 × 10-2 to 4 × 10-7, depending on the CMB subtraction algorithm and treatment of residual systematic errors; correcting for residual systematic errors consistently increases the significance of the effect. Within the target sample, the LOS decorrelation effect is stronger for sightlines with more misaligned magnetic fields, as expected. With our sample, we estimate that an intrinsic variation of ~15% in the ratio of 353 to 217 GHz polarized emission between clouds is sufficient to reproduce the measured effect. Our finding underlines the importance of ongoing studies to map the three-dimensional structure of the magnetized and dusty ISM that could ultimately help component separation methods to account for frequency decorrelation effects in CMB polarization studies.
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| 2. |
- Pelgrims, Vincent, et al.
(författare)
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Starlight-polarization-based tomography of the magnetized ISM: PASIPHAE's line-of-sight inversion method
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 670
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Tidskriftsartikel (refereegranskat)abstract
- We present the first Bayesian method for tomographic decomposition of the plane-of-sky orientation of the magnetic field with the use of stellar polarimetry and distance. This standalone tomographic inversion method presents an important step forward in reconstructing the magnetized interstellar medium (ISM) in three dimensions within dusty regions. We develop a model in which the polarization signal from the magnetized and dusty ISM is described by thin layers at various distances, a working assumption which should be satisfied in small-angular circular apertures. Our modeling makes it possible to infer the mean polarization (amplitude and orientation) induced by individual dusty clouds and to account for the turbulence-induced scatter in a generic way. We present a likelihood function that explicitly accounts for uncertainties in polarization and parallax. We develop a framework for reconstructing the magnetized ISM through the maximization of the log-likelihood using a nested sampling method. We test our Bayesian inversion method on mock data, representative of the high Galactic latitude sky, taking into account realistic uncertainties from Gaia and as expected for the optical polarization survey PASIPHAE according to the currently planned observing strategy. We demonstrate that our method is effective at recovering the cloud properties as soon as the polarization induced by a cloud to its background stars is higher than ~0.1% for the adopted survey exposure time and level of systematic uncertainty. The larger the induced polarization is, the better the method's performance, and the lower the number of required stars. Our method makes it possible to recover not only the mean polarization properties but also to characterize the intrinsic scatter, thus creating new ways to characterize ISM turbulence and the magnetic field strength. Finally, we apply our method to an existing data set of starlight polarization with known line-of-sight decomposition, demonstrating agreement with previous results and an improved quantification of uncertainties in cloud properties.
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| 3. |
- Skalidis, Raphael, et al.
(författare)
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CO enhancement by magnetohydrodynamic waves. Striations in the Polaris Flare
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 673
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Tidskriftsartikel (refereegranskat)abstract
- Context. The formation of molecular gas in interstellar clouds is a slow process, but can be enhanced by gas compression. Magneto-hydrodynamic (MHD) waves can create compressed quasi-periodic linear structures, referred to as striations. Striations are observed at the column densities at which the transition from atomic to molecular gas takes place. Aims: We explore the role of MHD waves in the CO chemistry in regions with striations within molecular clouds. Methods: We targeted a region with striations in the Polaris Flare cloud. We conducted a CO J = 2−1 survey in order to probe the molecular gas properties. We used archival starlight polarization data and dust emission maps in order to probe the magnetic field properties and compare against the CO morphological and kinematic properties. We assessed the interaction of compressible MHD wave modes with CO chemistry by comparing their characteristic timescales. Results: The estimated magnetic field is 38-76 µG. In the CO integrated intensity map, we observe a dominant quasiperiodic intensity structure that tends to be parallel to the magnetic field orientation and has a wavelength of approximately one parsec. The periodicity axis is ~17° off from the mean magnetic field orientation and is also observed in the dust intensity map. The contrast in the CO integrated intensity map is ~2.4 times higher than the contrast of the column density map, indicating that CO formation is enhanced locally. We suggest that a dominant slow magnetosonic mode with an estimated period of 2.1-3.4 Myr and a propagation speed of 0.30-0.45 km s−1 is likely to have enhanced the formation of CO, hence created the observed periodic pattern. We also suggest that within uncertainties, a fast magnetosonic mode with a period of 0.48 Myr and a velocity of 2.0 km s−1 could have played some role in increasing the CO abundance. Conclusions: Quasiperiodic CO structures observed in striation regions may be the imprint of MHD wave modes. The Alfvénic speed sets the dynamical timescales of the compressible MHD modes and determines which wave modes are involved in the CO chemistry.
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