Left: modified version of Fig. 5a of Tang et al. (2009). The magnetic field (red segments) detected with the SMA (angular resolution that at 5.4 kpc corresponds to  ~ 4000 AU) is superimposed on the 870 μm continuum contour map of W51–e2. The dashed black line indicates the direction of the ionized accreting flow by Keto & Klaassen (2008). Right: the magnetic field (red segments) detected with the SMA is superimposed on the 870 μm continuum contour map of W51–e2 (Tang et al., priv. comm.). In both images the green segments, which are scaled logarithmically according to polarization fraction P_l, mark the direction of the magnetic fields as derived from the linearly polarized emission of the CH₃OH masers (blue triangles; angular resolution corresponding to  ~ 5 AU).