OH Zeeman measurements of the magnetic fields in four megamaser galaxies

We report accurate circular polarization measurements of four bright OH megamasers (III Zw 35, 04454 - 4838, 10039 - 3338 and 20100 - 4156) with the Australia Telescope Compact Array. No convincing Zeeman splitting was observed. We place 3 sigma upper limits of approximately 3 mG on any uniform line-of-sight magnetic field present in the emitting regions. If the field is non-uniform, then we can place upper limits of approximately 5 mG. These limits in turn imply constraints on the physical conditions of the maser emission regions which lie in the central 100 pc of megamaser galaxies.