3D B-fieLds in the InterStellar medium and Star-forming regions (3D-BLISS): I. Using Starlight Polarization in Massive IRDC Filament G11.11-0.12

TL;DR

This work introduces and applies a 3D B-field diagnostic (3D-BLISS) that combines starlight polarization with MRAT alignment to recover the inclination of magnetic fields along the line of sight in a massive IRDC filament. By constraining dust properties (e.g., $\langle a_{max} \rangle \approx 0.25\ \mu$m and axial ratio $s \gtrsim 1.4$) from Gaia extinction curves and Ks-band polarization data, the authors infer mean B-field inclinations of about $|\gamma_{obs}| \sim 50^{\circ}$ and reveal an arc-shaped 3D B-field morphology in G11.11-0.12, with outer regions showing larger inclinations and inner parts closer to the filament spine. The inferred 3D B-field strengths ($B_{3D} \sim 80$–$150\ \mu$G) imply substantial magnetic support, yielding sub- to trans-Alfvénic regimes in outer regions and near-critical conditions in dense zones, thereby underscoring the role of 3D geometry in filament evolution. The study demonstrates the feasibility of combining starlight polarization, dust physics, and MRAT theory to quantify 3D B-fields across multi-scale environments, setting the stage for coordinated, multi-wavelength follow-up of filament formation and evolution.

Abstract

Three-dimensional magnetic fields (3D B-fields) are essential to understand the formation and evolution of the interstellar medium and multi-scale star formation; however, the accurate measurement of 3D B-fields is still challenging. The angle of dust polarization by magnetically aligned grains provides the projected B-fields onto the plane-of-sky, while the degree of dust polarization provides the B-field's inclination angles with respect to the line-of-sight. Our previous theoretical studies proposed a new method of probing 3D B-fields using dust polarization combined with the Radiative Torque (RAT) alignment theory and demonstrated the accurate inference of B-field inclination angles using synthetic polarization data. In this paper, we report the first application of the new technique to study 3D B-fields and dust properties in the G11.11-0.12 filament (hereafter G11) from starlight polarization observations taken by ISRF/SIRPOL at $2.19\,\rmμm$. Using both observed starlight polarization and optical dust extinction curve from Gaia mission, we constrained the maximum grain size of $0.25\,\rmμm$ and the grain elongation with an axial ratio of $s\gtrsim 1.4$ in the outer regions of G11. We calculated the alignment properties in the G11 by using the \textsc{DustPOL\_py} code based on the RAT theory. The B-field's inclination angles are then inferred from the observed starlight polarization efficiency when the grain alignment is included, showing the inclined B-fields in the G11 with a mean angle of $\sim 50$ degrees. From these inferred inclination angles, we found evidence of the local 3D arc-shaped B-field structure toward the sightline. These findings are important for fully understanding 3D B-field's roles in the formation and evolution of massive filamentary clouds.

Figures (16)

• Figure 1: The foreground-corrected $K_{s}$-band polarization measured by SIRPOL at $2.19\,\rm\mu m$ (blue segments, see Chen2023), also indicating the orientation of POS B-fields in the outer regions of G11. The length of the blue segments is in an arbitrary scale. The polarization vectors are overplotted onto a gray map of molecular hydrogen column density $N_{\rm H_2}$ derived from Herschel observations (Zucker2018_filament). The massive clumps P1 and P6 are marked by yellow stars (Henning2010). The red contours indicate the column density at 1.5, 2, and 2.5 $\times 10^{22}\,\rm\,{\rm cm}^{-2}$. The spine structure of the G11 filament is indicated by reg segments. The G11 filament is divided into four sub-regions (Regions A, B, C, and D) along the spine in the Galactic coordinate.
• Figure 2: The starlight polarization efficiency $P_{\rm K}/N_{\rm H}$ vs. the molecular hydrogen column density $N_{\rm H_2}$ in four sub-regions along the spine of the G11 filament. A power-law fit is applied to the running mean of the observed data.
• Figure 3: The spatial distribution of the magnetic turbulence factor $F_{\rm turb}$ calculated from the polarization angle dispersion in each grid cell spacing of $2' \times 2 '$ (see also Chen2023) by using the unsharp-masking method (Pattle2017).
• Figure 4: The spatial distribution of the total-to-selective extinction ratio $R_{\rm V}$ in the outer regions of G11 derived from interstellar dust extinction data from background stars provided by the Gaia mission (Zhang2025). The observed $R_{\rm V}$ is around 2.65 - 2.95
• Figure 5: The variation of the modeled $R_{\rm V}$ generated by DustPOL_py for Astrodust+PAHs grains, considering varying maximum grain size $a_{\rm max}$ and grain axial ratio of oblate spheroids $s > 1$. The $R_{\rm V}$ increases with increasing $a_{\rm max}$. The modeled $R_{\rm V}$ is best-fitted to the observed one when the mean $\langle a_{\rm max} \rangle = 0.25\,\rm\mu m$.