SIMPLIFI—Study of Interstellar Magnetic Polarization: A Legacy Investigation of Filaments. I. Magnetically Guided Accretion onto the DR21 Ridge

Abstract We present first results from SIMPLIFI (Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments), a SOFIA/HAWC+ 214 μ m polarimetric survey of Galactic molecular cloud filaments. We trace magnetic field morphology from the DR21 Main Ridge into surrounding subfilaments at ∼0.1 pc resolution, extending polarimetric detections for the first time beyond high-column-density regions probed by prior submillimeter observations. We compare the plane-of-sky orientations of the magnetic field, B ˆ pos , the projected gravitational acceleration, g pos , and the intensity gradient rotated by 90°, ∇ ˆ ⊥ I . The relative orientation of B ˆ pos and ∇ ˆ ⊥ I transitions from preferentially parallel in subfilaments to perpendicular in the DR21 Main Ridge at N (H 2 ) ∼ 2 × 10 22 cm −2 , consistent with thresholds seen with Planck and expected in clouds formed from strongly magnetized, sub-Alfvénic, magnetically subcritical gas ( M A ≲ 1 , M / Φ B < [ M / Φ B ] cr ). We find that the relative alignment between orientations shows region-to-region and pixel-to-pixel variations at fixed column density. Column density alone is thus not sufficient to encode changes in magnetic field structure. Theoretical models must account for additional drivers. Our central finding is that g pos and B