SEARCH FOR LINEAR-POLARIZATION OF EXTREME HIGH-VELOCITY WATER MASERS IN NGC-4258

We have measured the upper limit of linear polarization of water maser emission from NGC 4258. Water masers in NGC 4258 are composed of a broad central component of width about 100 km s-1 and spiky extreme high velocity components on both higher and lower velocity sides of the central component by about 1000 km s-1. Several possible models for the extreme components have been proposed and may predict observable amounts of linear polarization unless Faraday depolarization quenches the linear polarization in the amplification process. We have found that the polarization degrees are less than about 20% for the extreme components and less than about 15% for the central velocity component. This finding suggests, under the restriction in some models, that strong Faraday rotation may be acting in the maser clouds to quench the linear polarization. The magnetic field strength required in two models is estimated. In each model, the estimated value does not exceed by much the value computed from magnetic flux conservation in spherical collapse of general interstellar clouds. We have also observed the water maser flare of the irregular galaxy IC10 and found no linear polarization in the flare component. © 1995 American Institute of Physics.