The JCMT BISTRO-2 Survey: Magnetic Fields of the Massive DR21 Filament

We present 850 $\mu$m dust polarization observations of the massive DR21 filament from B-fields In STar-forming Region Observations (BISTRO) survey, using POL-2 polarimeter and SCUBA-2 camera on James Clerk Maxwell Telescope. detect ordered magnetic fields perpendicular to parsec-scale ridge main filament. sub-filaments, are mainly parallel filamentary structures smoothly connect compare Planck study field 0.1--10 pc scales. The revealed in data well aligned with those data, indicating a smooth variation large small plane-of-sky strengths derived angular dispersion functions 0.6--1.0 mG $\sim$ 0.1 surrounding ambient gas. mass-to-flux ratios found be magnetically supercritical slightly subcritical nearly critical alignment between column density changes random low-density gas probed by mostly high-density JCMT. agreement MHD simulations strongly magnetized medium, suggesting that play an important role shaping sub-filaments.