Very Long Baseline Interferometry (VLBI) observations of water maser galaxies have revealed a rotating disk in the nuclear region of NGC 4258. With its linear size and Keplerian rotation of the disk, the existence of a massive black hole is concluded at the center of the disk. In addition, this rotating system provides a new tool to measure accurate distance to the galaxy. Accurate mass of the black hole and X-ray observation enable us to estimate accuration rate on to the black hole, which turns out to be very small. After this discovery, maser structure of several galaxies has been investigated with VLBI. VLBI observations of these maser galaxies are briefly reviewed and discussed.
展开▼
