Observational and theoretical studies of high-velocity components of extragalactic water masers

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In 1992, water masers in NGC4258 was discoverd to exhibit extreme high velocity features of about 1000km/s, which is much larger than the outflow velocity of normal water masers. For the explanation of this phenomena, we have studied the models of masers from a rotating disk around the galactic nucleus, or models of the frequncy shift due to Raman scattering, both observationally and theoretically. First, we have investigated the model of Raman scattering in plasma and found what conditions are required for the density and temperature of the plasma (Deguchi 1994). It was predicted in this paper that, if this model is correct, emission from extreme high velocity components in NGC4258 is slightly polarized. We have performed the polarization observationswith the 37m radio telescope at Haystack, U.S.A.It has turned out in two sessions of observations that the degree of linear polarization is less than 20% (Deguchi 1995). This reult indicates that the linear polarization disappears due to the Farady depolarization in the strong magnetic field. Miyoshi et al. (1995) have performed position measurements of extreme velocity components with U.S.Very Long Baseline Array and found that the extreme components come from the position by 3 milli-arcsec separated from the main components. Though Watson and Walin proposed a rotating disk model which can provide the condition between positions and velocities of spots, we have shown that the simple disk model have to add the warp motion of the disk rotation and it cannot explain the weakness of the blue -shifted feature. In addition, to investigate the general properties of astrophysiccal masers, we have investigated the pumping mechanism of masers (especially for the methanol masers) with Dr.A.M.Sobolev. We have also worked on the Raman scattering masers with Dr.R.T.Gangadhara in Indian Institute of Astrophysics.