Interstellar Polarization Survey. V. Galactic Magnetic Field Tomography of the Spiral Arms Using Optical and Near-infrared Starlight Polarization

Abstract Interstellar linear polarization occurs when starlight passes through elongated dust grains aligned by interstellar magnetic fields. The observed polarization can come from different dust structures along the line of sight (LOS). By combining polarization measurements with stellar distances, we can study the plane-of-sky Galactic magnetic field (GMF) between the observer and the star and separate the contributions of clouds with different GMF properties. We used optical and near-infrared (NIR) polarization data from three regions in the Galactic plane (∣ b ∣ &lt; 1° and 19 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 8 &lt; l &lt; 25 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 5) to perform a polarization decomposition across the Galactic arms. A comparison between the optical and NIR data showed an optical-to-NIR polarization ratio of two to three along the LOS and a consistent polarization angle across both wavelengths in all studied regions, within the measurement uncertainties. We applied the Bayesian Inference of Starlight Polarization in one dimension and the Gaussian mixture model methods to decompose the polarization in the three regions. The optical and NIR observations complemented each other, consistently identifying nearby ( d ≲ 143 pc), intermediate (0.47 kpc &lt; d &lt; 1.2 kpc), and distant (1.5 kpc &lt; d &lt; 2.5 kpc) polarizing clouds, in agreement with previous findings of the Local Bubble wall, the Local Arm, and Sagittarius Arm dust structures. The results from both polarization decomposition methods agree and complement each other. Polarization tomography revealed significant LOS variations in the plane-of-sky magnetic field orientation in two of the three regions. The relative alignment between the magnetic fields traced by starlight polarization and Planck’s polarized thermal dust emission at 353 GHz reaffirmed these variations.