May 13, 1999 CfA Colloquium


Title: The NGC4258 Water Maser: A Unique Window into the Sub-Parsec-Scale World of an Active Galactic Nucleus

Speaker: James Herrnstein

Abstract: Abstract: It has been more than four years since the first VLBI images of the NGC4258 water maser provided striking evidence for the existence of supermassive black holes in the nuclei of active galaxies. In the intervening years, we have pursued an aggressive program of interferometric observations of the source. In this talk I will concentrate on two results recently extracted from these data. The first is the successful detection of 30 muas/yr proper motions amongst the NGC4258 masers, and the derivation of a geometric distance to the galaxy from these motions. I will discuss the maser distance in the context of the extragalactic distance ladder, arguing that the former is destined to play a significant role in calibrating the latter. The second result concerns the determination of the complete three-dimensional geometry of the warped accretion disk in the nucleus of NGC4258. I will show that the details of this geometry are likely to have important ramifications on theories of Seyfert unification, as well as on our understanding of active galaxy accretion processes.

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