Demonstration of magnetic field tomography with starlight polarization towards a diffuse sightline of the ISM

TL;DR

This paper demonstrates a method to reconstruct the three-dimensional magnetic field structure of the interstellar medium using stellar polarization data combined with Gaia distances, revealing distinct magnetic properties of two interstellar clouds.

Contribution

It introduces a novel tomographic decomposition technique to analyze line-of-sight polarized starlight, enabling the separation of magnetic field properties of multiple interstellar clouds.

Findings

Two clouds along the line of sight have different polarization properties.

The magnetic field orientation differs by 60 degrees between the two clouds.

The method can constrain cloud distances and polarization efficiencies.

Abstract

The availability of large datasets 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 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 kpc from the Sun. HI 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 degrees. 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.