Starlight-polarization-based tomography of the magnetized ISM: PASIPHAE’s line-of-sight inversion method

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.

Tomographic Imaging of the Sagittarius Spiral Arm's Magnetic Field Structure

“…The scattered distribution of dust clouds and their discrete contribution to the polarization (Figure 4) is comparable to the finding for the Perseus and (foreground) Taurus molecular clouds (Doi et al 2021b) and the thin-layer model developed for high Galactic latitude clouds (Pelgrims et al 2023). This suggests that the thin-layer model is also applicable to LOSs at low Galactic latitudes.…”

“…To accurately evaluate the magnetic field structure associated with each cloud, it is important to consider that the observed polarization is a result of integrating all contributions along the optical path to the stars. The relative Stokes parameters q Gal and u Gal can be approximated as an addition of the contributions from each element along the LOS, particularly in the case of low polarization levels (say, =10%; e.g., Patat et al 2010;Panopoulou et al 2019;Pelgrims et al 2023). By subtracting the foreground contribution from the observed polarization in each distance range, we can obtain a more reliable approximation of the intrinsic magnetic field structure associated with each cloud.…”

“…We note, however, that it is important for future observations to increase the sample size and determine the precise shape of the σ q,u distributions, as these distributions contain information about the PA distribution of turbulent magnetic fields and the spatial variation of dust properties; the σ q,u value measured in the direction perpendicular to each cloud's mean q-u vector (hereafter σ q,u⊥ ) approximates  PA s and reflects the PA dispersion of the magnetic field on the POS, and the σ q,u value measured in the direction parallel to each cloud's mean q-u vector (hereafter σ q,u∥ ) is considered to arise from the angular dispersion in the LOS direction of the magnetic field, as well as from fluctuations in polarization efficiency for each region within the polarizing cloud (such as variations in column density and dust alignment efficiency; also see Pelgrims et al 2023).…”

“…In recent years, Gaia data have provided accurate distances to stars (Gaia Collaboration et al 2016Bailer-Jones et al 2021) and interstellar extinction values for these stars (Andrae et al 2023;Babusiaux et al 2023). By combining these pieces of information with stellar polarimetry data, it becomes possible to reveal the 3D distribution of the ISM and its associated magnetic field up to distances of a few kiloparsecs (e.g., Panopoulou et al 2019;Doi et al 2021b;Pelgrims et al 2023).…”

WALOP-South: a four-camera one-shot imaging polarimeter for PASIPHAE survey. Paper II – polarimetric modeling and calibration

“…Using the two upcoming Wide-Area Linear Optical Polarimeters 1 (WALOPs) as survey instruments, the Polar-Areas Stellar Imaging in Polarization High Accuracy Experiment (PASIPHAE) program aims to create the first large swath (> 4000 square degrees) optopolarimetric map of the sky, towards the Galactic polar regions. The main objectives of the PASIPHAE program include -(i) determining the 3D structure of the dust distribution along a large number of lines of sight with the sub-degree plane of sky angular resolution, (ii) determining the plane of the sky orientation of the magnetic fields associated with the multiple dust clouds that are generally seen along each line of sight, [2][3][4][5] (iii) to test the physics of interstellar dust, especially concerning grain alignment and size distribution, and (iv) to trace paths traversed by ultra-high-energy cosmic rays through the Galaxy. For an extensive description of the motivation and scientific objectives of the PASIPHAE program, we refer the reader to the PASIPHAE white paper by Tassis et al 6 The survey will be concurrently executed from the southern and northern hemispheres, using the WALOP-South instrument mounted on the 1 m telescope at South African Astronomical Observatory's (SAAO) Sutherland Observatory and WALOP-North mounted on the 1.3 m telescope at Skinakas Observatory, Greece, respectively.…”