Demonstration of Magnetic Field Tomography with Starlight Polarization toward a Diffuse Sightline of the ISM

Abstract

The availability of large data sets with stellar distance and polarization information will enable a tomographic reconstruction of the (plane-of-the-sky-projected) interstellar magnetic field in the near future. We demonstrate the feasibility of such a decomposition within a small region of the diffuse interstellar medium (ISM). We combine measurements of starlight (R-band) linear polarization obtained using the RoboPol polarimeter with stellar distances from the second Gaia data release. The stellar sample is brighter than 17 mag in the R-band and reaches out to several kiloparsecs from the Sun. H I emission spectra reveal the existence of two distinct clouds along the line of sight. We decompose the line-of-sight-integrated stellar polarizations to obtain the mean polarization properties of the two clouds. The two clouds exhibit significant differences in terms of column density and polarization properties. Their mean plane-of-the-sky magnetic field orientation differs by 60°. We show how our tomographic decomposition can be used to constrain our estimates of the polarizing efficiency of the clouds as well as the frequency dependence of the polarization angle of polarized dust emission. We also demonstrate a new method to constrain cloud distances based on this decomposition. Our results represent a preview of the wealth of information that can be obtained from a tomographic map of the ISM magnetic field.

Mapping the Magnetic Interstellar Medium in Three Dimensions over the Full Sky with Neutral Hydrogen

“…These values agree well with the polarimetry-derived measurements for the two clouds. We find a higher p H i for the Stokes maps integrated over the LVC velocity range than for the IVC velocity range, just as Panopoulou et al (2019) derive a higher fractional linear polarization associated with the LVC than with the IVC.…”

“…The texture overlaid on the maps is our map of θH i at 30 , computed from QH i and UH i integrated over the velocity regions highlighted in the top panels and visualized using line integral convolution (Cabral & Leedom 1993). Orange line segments show the Panopoulou et al (2019) measurements of the local magnetic field orientation: those authors find that the IVC is located at a distance of 1250 − 2140 pc and has a mean magnetic field orientation θ = 106 • ± 8 • , and the LVC is at a distance of 346 − 393 pc with θ = 42.6 • ± 1 • . Averaged over the two-cloud region, we find θH i = 111.6 • for the IVC and θH i = 42.6 • for the LVC, in excellent agreement with the Panopoulou et al (2019) values. parameters derived from H i emission, but not a unique solution.…”

“…To compare our three-dimensional Stokes parameter maps with the Panopoulou et al (2019) polarimetric analysis, we select the velocity range in our maps closest to the ranges associated with the IVC and LVC: these are −51.4 < v < −38.5 km s −1 and −3.8 < v < −1.2 km s −1 , respectively. We compute Q H i and U H i from Equations 10 and 11 over these velocity ranges, and visualize the resulting magnetic field orientation θ H i in the two regions in Figure 17.…”

“…A comparison between our velocity-resolved maps and the distance-decomposed magnetic field orientation estimate from Panopoulou et al (2019). In their tomographic analysis, Panopoulou et al (2019) used starlight polarization measurements in the "two-cloud" region indicated by the white dotted line, a line of sight that contains H i emission from two prominent features, an intermediate velocity cloud (IVC) and a local velocity cloud (LVC). Their analysis also utilized a "control" region with mostly local emission, indicated by the dashed red line.…”

An Imprint of the Galactic Magnetic Field in the Diffuse Unpolarized Dust Emission

“…While historically starlight polarization has been and continues to be a powerful probe of magnetic fields in the interstellar medium (e.g., Clemens et al 2012;Planck Collaboration Int. XXI 2015;Panopoulou et al 2019), the relatively recent advent of sensitive ground-based, stratospheric, and space-based probes of polarized far-infrared (FIR) dust emission has opened new windows to study magnetic fields in molecular clouds, protoplanetary disks, and the diffuse ISM (e.g., Benoît et al 2004;Galitzki et al 2014;Stephens et al 2014;Planck Collaboration Int. XIX 2015;Ward-Thompson et al 2017;Chuss et al 2019).…”