AAS 205th Meeting, 9-13 January 2005
Session 73 Distances, the CMB and the Sunyaev-Zeldovich Effect
Oral, Tuesday, January 11, 2005, 10:00-11:30am, Golden Ballroom

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[73.01] Improved Maser Distance to NGC 4258

E. M. L. Humphreys, L. J. Greenhill, M. J. Reid, A. L. Argon, J. M. Moran (Harvard-Smithsonian CfA)

NGC 4258 is a nearby Seyfert II galaxy notable for intense water maser emission originating from the circumnuclear accretion disk, at < 0.3 pc from a supermassive black hole. Water vapor masers orbit the black hole in almost perfect Keplerian rotation, tracing sub-pc portions of the disk around an AGN central engine. The relatively simple geometry of the source, and the well-ordered kinematics, enable estimation of a distance that is free from the calibration and systematic uncertainties intrinsic to other distance indicators. Herrnstein et al. (1999) used 4 epochs of VLBA data over 3 years to estimate a geometric distance to NGC 4258 of 7.2 Mpc ±6% ±4% (systematic and random error respectively). We have extended the monitoring to over 40 epochs of VLBA, VLA, Effelsberg and GBT data over 10 years, reducing the random error by 2. We have performed detailed modeling of the disk to investigate the systematic error by modeling eccentric orbits of masers, and to provide a unique evaluation of the geometry of an AGN accretion disk. We expect to reduce the total error in the distance estimate to NGC 4258 by a factor of about 2. A geometric estimate of distance to this galaxy may be compared to estimates obtained from Cepheid variable stars and in the context of the recently completed HST Key Project, used to re-evaluate H0. This forms one of three talks that describe distance estimates to NGC 4258 and ramifications for the extragalactic distance scale. The research has been funded in part by NASA grant NAG05-10311.

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