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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. 

Abstract Star formation primarily occurs in filaments where magnetic fields are expected to be dynamically important. The largest and densest filaments trace the spiral structure within galaxies. Over a dozen of these dense (∼10 4 cm −3 ) and long (>10 pc) filaments have been found within the Milky Way, and they are often referred to as “bones.” Until now, none of these bones has had its magnetic field resolved and mapped in its entirety. We introduce the SOFIA legacy project FIELDMAPS which has begun mapping ∼10 of these Milky Way bones using the HAWC+ instrument at 214 μ m and 18.″2 resolution. Here we present a first result from this survey on the ∼60 pc long bone G47. Contrary to some studies of dense filaments in the Galactic plane, we find that the magnetic field is often not perpendicular to the spine (i.e., the center line of the bone). Fields tend to be perpendicular in the densest areas of active star formation and more parallel or random in other areas. The average field is neither parallel nor perpendicular to the Galactic plane or the bone. The magnetic field strengths along the spine typically vary from ∼20 to ∼100 μ G. Magnetic fields tend to be strong enough to suppress collapse along much of the bone, but for areas that are most active in star formation, the fields are notably less able to resist gravitational collapse.