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Abstract Stars primarily form in galactic spiral arms within dense, filamentary molecular clouds. The largest and most elongated of these molecular clouds are referred to as “bones,” which are massive, velocity-coherent filaments (lengths ∼20 to >100 pc, widths ∼1–2 pc) that run approximately parallel and in close proximity to the Galactic plane. While these bones have been generally well characterized, the importance and structure of their magnetic fields (B-fields) remain largely unconstrained. Through the Stratospheric Observatory for Infrared Astronomy Legacy program FIlaments Extremely Long and Dark: a Magnetic Polarization Survey (FIELDMAPS), we mapped the B-fields of 10 bones in the Milky Way. We found that their B-fields are varied, with no single preferred alignment along the entire spine of the bones. At higher column densities, the spines of the bones are more likely to align perpendicularly to the B-fields, although this is not ubiquitous, and the alignment shows no strong correlation with the locations of identified young stellar objects. We estimated the B-field strengths across the bones and found them to be ∼30–150μG at parsec scales. Despite the generally low virial parameters, the B-fields are strong compared to the local gravity, suggesting that B-fields play a significant role in resisting global collapse. Moreover, the B-fields may slow and guide gas flow during dissipation. Recent star formation within the bones may be due to high-density pockets at smaller scales, which could have formed before or simultaneously with the bones.
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This content will become publicly available on December 15, 2026
FIELDMAPS Data Release: Far-infrared Polarization in the “Bones” of the Milky Way
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 214μm toward 10 filamentary candidate “bones” in the Milky Way using the High-resolution Airborne Wide-band Camera on the Stratospheric Observatory for Infrared Astronomy. These data were taken as part of the Filaments Extremely Long and Dark: A Magnetic Polarization Survey 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 10 candidate bones, and we find a mean difference and standard deviation of −74° ± 32° 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 850μm polarization data from Planck on scales greater than 10 pc show a nearly parallel mean difference of 3° ± 21°. 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 ± 0.1. The polarization, dust temperature, and column density data presented in this work are publicly available online.
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- PAR ID:
- 10656001
- Publisher / Repository:
- The Astrophysical Journal Supplement Series
- Date Published:
- Journal Name:
- The Astrophysical Journal Supplement Series
- Volume:
- 282
- Issue:
- 1
- ISSN:
- 0067-0049
- Page Range / eLocation ID:
- 2
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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