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P. T. Kondratko, L. J. Greenhill, J. M. Moran (CfA)
The H2O maser in NGC 4258 traces a thin, almost edge-on, and slightly warped molecular disk around a supermassive black hole. Extensive measurements of the positions, three dimensional velocity vectors, and accelerations of dozens of masers are well explained by a simple disk model. However, discovery of two quasars with redshifts 0.39 and 0.65 symmetrically displaced by ~9' from the nucleus of NGC 4258 and located approximately at the position angle of the maser disk has fueled an alternative hypothesis by G. Burbidge, E.M. Burbidge, and co-workers. They proposed that the two quasars were ejected from the nucleus, are dynamically associated with the maser, and possess redshifts which are primarily non-cosmological in origin. We use 3 epochs of 20 cm VLBA images of one of the quasars and of NGC 4258 to derive an upper limit on the quasar proper motion of 0.4 mas year-1 (0.05 c) in the putative direction of ejection and with respect to the continuum emission from NGC 4258. This upper limit excludes the Ozernoy model in which the redshifts of the two quasars are primarily due to the transverse Doppler effect caused by a tranverse velocity of 6.6 mas year-1 (0.75 c). In addition to fixing an upper limit on the quasar proper motion, the images of NGC 4258 jet reveal a dominant, apparently stationary component located approximately ~ 6 mas north from the black hole and corresponding to the northern core of the jet. The distance of the core from the black hole scales with \lambda as expected in the standard model of Blandford and Königl. The southern jet core is undetected perhaps due to high optical depth associated with a hot, ionized wind or with an ionized material in the disk. The jet in the immediate vicinity of the central engine is characterized by \gamma>4.3, which appears to be relatively stable over time.
The author(s) of this abstract have provided an email address for comments about the abstract: pkondrat@cfa.harvard.edu