H i Zeeman Measurements of the Magnetic Field in Sagittarius B2

We have measured the line-of-sight magnetic field toward the Sgr B2 North and Main regions with the VLA by Zeeman mapping of the H I line; Blos ~ -0.5 mG with spatial variations of about 50%. It is likely that the H I absorption line is produced mainly within the massive molecular envelope surrounding the dense Sgr B2 cores; if so, the Blos we measure is that of the Sgr B2 molecular cloud complex. Maps of the plane-of-sky magnetic field direction from polarized emission of dust have led others to suggest that the present north-south elongation of the Sgr B2 cloud was due to infall along the predominantly east-west oriented magnetic field lines. Our measured magnetic field strength is approximately equal to the critical magnetic field required to support the entire cloud complex against gravitational collapse and thus provides support for this picture. Ambipolar diffusion heating may explain the high gas temperatures in the Sgr B2 cloud.