FIELDMAPS Data Release: Far-Infrared Polarization in the "Bones" of the Milky Way

TL;DR

The paper presents FIELDMAPS, a uniform, high-resolution survey of polarized dust emission at 214 μm in ten Milky Way bones, revealing magnetic fields that are, on average, perpendicular to filament lengths at parsec scales, in contrast to Planck’s larger-scale, disk-parallel field. Using SOFIA/HAWC+ data complemented by Herschel, Planck, and Spitzer observations, the authors derive polarization properties, map magnetic-field structures, and quantify the relationship between polarization and intensity via a power-law index around $\alpha \approx -0.7$. They show that magnetic fields in bones are locally well-ordered but vary along the filaments, and discuss implications for dust grain alignment, radiative torques, and the role of magnetic support in star-forming filaments. The work emphasizes the need for multi-wavelength, high-resolution polarization studies to disentangle projection effects, grain alignment physics, and environmental factors, and provides a public data release to enable future analyses and cross-surveys.

Abstract

Polarization observations of the Milky Way and many other spiral galaxies have found a close correspondence between the orientation of spiral arms and magnetic field lines on scales of hundreds of parsecs. This paper presents polarization measurements at 214 $μ$m toward ten filamentary candidate ``bones" in the Milky Way using the High-resolution Airborne Wide-band Camera (HAWC+) on the Stratospheric Observatory for Infrared Astronomy (SOFIA). These data were taken as part of the Filaments Extremely Long and Dark: A Magnetic Polarization Survey (FIELDMAPS) and represent the first study to resolve the magnetic field in spiral arms at parsec scales. We describe the complex yet well-defined polarization structure of all ten candidate bones, and we find a mean difference and standard deviation of $-74^{\circ} \pm 32^{\circ}$ between their filament axis and the plane-of-sky magnetic field, closer to a field perpendicular to their length rather than parallel. By contrast, the 850 $μ$m polarization data from \textit{Planck} on scales greater than 10 pc show a nearly parallel mean difference of $3^{\circ} \pm 21^{\circ}$. These findings provide further evidence that magnetic fields can change orientation at the scale of dense molecular clouds, even along spiral arms. Finally, we use a power law to fit the dust polarization fraction as a function of total intensity on a cloud-by-cloud basis and find indices between $-0.6$ and $-0.9$, with a mean and standard deviation of $-0.7 \pm 0.1$. The polarization, dust temperature, and column density data presented in this work are publicly available online.

Figures (32)

• Figure 1: Spitzer red (24 $\mu$m), green (8.0 $\mu$m), and blue (3.6 $\mu$m) composite image for Filament 4. The color levels were chosen to highlight the infrared-dark features of the bone. Class I and Flat Spectrum (red), Class II (blue), and Class III (white) YSOs from the literature are identified with circles. The three contours trace Herschel-derived $N_{H_2}$ column densities of $0.5$, $1.0$, and $1.5 \times 10^{22}$ cm$^{-2}$, with the lowest level identified in orange for clarity. The Herschel beam at 500 $\mu$m is given by the gray circle at the bottom left. A reference length of 1 pc is provided at the top right, assuming a distance $D_\odot$ of 4.5 kpc to the cloud (see Table \ref{['tab:filaments']}).
• Figure 2: Filament 1. Top: The inferred magnetic field structure (HAWC+ 214 $\mu$m: red, Planck 850 $\mu$m: blue) plotted on the Herschel-derived $N_{H_2}$ column density map (gray). The magnetic field angle $\theta_B$ is obtained from rotating the polarization angle $\theta$ by $90^\circ$. Class I (green), Flat Spectrum (violet), and Class II (blue) YSOs are identified with star symbols. The gray contour denotes the area observed by HAWC+. The circles denote the beam sizes of HAWC+ (red) and Herschel (green). Bottom: The polarization measurements (red) plotted on the HAWC+ Stokes $I$ total intensity map (gray) at 214 $\mu$m. The vector length shows the polarization fraction $P$. For each panel, only every third vector is plotted for both SOFIA and Planck data, and the white contours trace the Stokes $I$ total intensity from 10 mJy arcsec$^{-2}$ and increasing by steps of 10 mJy arcsec$^{-2}$. See Section \ref{['sub:polmaps']} for details.
• Figure 3: Filament 2. Same as Figure \ref{['fig:Fil1_Maps']}, with every fourth vector plotted for both SOFIA and Planck data. See Section \ref{['sub:polmaps']} for details.
• Figure 4: Filament 4. Same as Figure \ref{['fig:Fil1_Maps']}, with every third vector plotted for both SOFIA and Planck data. See Section \ref{['sub:polmaps']} for details. This figure displays the same area as Figure \ref{['fig:Fil4_RGB']}.
• Figure 5: Filament 5. Same as Figure \ref{['fig:Fil1_Maps']}, with every third vector plotted for both SOFIA and Planck data. See Section \ref{['sub:polmaps']} for details.