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

We present first results from SIMPLIFI (Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments), a SOFIA/HAWC+ 214 μm 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, Bˆpos , the projected gravitational acceleration, gpos , and the intensity gradient rotated by 90°, ∇ˆ⊥I . The relative orientation of Bˆpos and ∇ˆ⊥I transitions from preferentially parallel in subfilaments to perpendicular in the DR21 Main Ridge at N(H2) ∼ 2 × 1022 cm−2, consistent with thresholds seen with Planck and expected in clouds formed from strongly magnetized, sub-Alfvénic, magnetically subcritical gas ( MA≲1 , M/ΦB<[M/ΦB]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 gpos and Bˆpos remain aligned throughout the cloud regardless of column density or environment, unlike the environment-dependent behavior of Bˆpos versus ∇ˆ⊥I and gpos versus ∇ˆ⊥I . This persistent alignment is consistent with magnetically guided accretion: subfilaments channel material along field lines at several 10−3 M⊙ yr−1, sufficient to assemble the Ridge within ∼106 yr and sustain high-mass star formation. This framework also explains why observed radial velocities (≈2 km s−1) fall well below freefall expectations (≈8 km s−1): with the field nearly in the plane of the sky, only a small fraction of the accretion velocity projects along the line of sight.