Towards an Understanding of the Dynamics of the Milky Way

Determination of outer rotation curve using Galactic masers The rotation curve of the Galaxy is poorly determined outside the solar circle (e.g. Honma & Sofue 1997). The situation makes difficult to derive the dynamical mass of the Galaxy, which is essential for inferring the amount of Galactic dark gravitating matter. We selected Galactic masers in this study since they distributed over the whole Galaxy. Moreover, they can observed with VLBI at extremely high angular resolution, allowing a measurement of the annual parallax, and thus the source’s distance, and its proper motion due to Galactic rotation with phase referencing VLBI astrometric observation. We have already performed phase-referencing VLBA observations between Galactic water masers and adjacent extragalactic continuum sources. We succeeded to detect the annual parallax of masers in W3(OH) which are located at 2 kpc away from the Sun. This result showed the feasibility for this line astrometric observation to kpc scale in the Galaxy. Although we detected the astrometric parameters of Galactic water maser sources, this limited sample of sources is insufficient to determine the outer Galactic rotation curve. To observe other water masers with the VLBA, we searched for possible candidates to attain of our scientific goal. We observed with the VLA a survey of 36 Galactic water maser sources in the Perseus arm and will survey over 50 candidate water maser sources beyond the Perseus arm with single-dish radio-telescope.