Moran, James M. 2008. "The Black-Hole Accretion Disk in NGC 4258: One of Nature’s Most Beautiful Dynamical Systems." In Frontiers of Astrophysics: A Celebration of NRAO’s 50th Anniversary ASP Conference Series, Vol. 395, eds. Alan H. Bridle, James J. Condon, and Gareth C. Hunt, 87-100. San Francisco, CA: Astronomical Society of the Pacific.
In this talk I will summarize some of the work that the CfA group has done to study the structure of the water masers in the accretion disk of NGC4258. A series of 18 epochs of VLBA data taken from 1997.3 to 2000.8 were used for this study. The vertical distribution of maser features in the systemic group was found to be Gaussian, as expected for hydrostatic equilibrium, with a σ-width of 5.1 microarcsec (μas). If the disk is in hydrostatic equilibrium, its temperature is about 600 K. The systemic features exhibit a small, but persistent, gradient in acceleration versus impact parameter. This characteristic may indicate the presence of a spiral density wave rotating at sub- Keplerian speed. A more precise understanding of the dynamical properties of the disk is expected to lead to a more refined estimate of the distance to the galaxy.