The magnetic field structure in W51A

We present 850 m imaging polarimetry of the W51A massive star forming region performed with SCUBA on the JCMT. From the polarimetry we infer the column-averaged magnetic eld direction, projected onto the plane of the sky. We nd that the magnetic eld geometry in the region is complicated. We compare the eld geometry with 6 cm and CS J =7 6 emission and determine that the magnetic eld must be relatively weak and plays a passive role, allowing itself to be shaped by pressure forces and dynamics in the ionised and neutral gases. Comparisons are drawn between our data and 1.3 mm BIMA interferometric polarimetry data, from which we conclude that the magnetic eld must increase in importance as we move to smaller scales and closer to sites of active star formation.