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L. J. Greenhill, A. L. Argon (CfA), D. Bersier (STScI), E. M. L. Humphreys (CfA), L. M. Macri (NOAO), J. M. Moran, M. J. Reid, K. Z. Stanek (CfA)
Observations of the Cosmic Microwave Background have been critical to estimation of parameters for lambda-CDM models. However, external constraints are needed to resolve degeneracies. Specifically, high-accuracy estimates of the Hubble constant (Ho) are needed to establish the extragalactic distance scale and obtain robust constraint on the equation of state of dark energy. Traditional techniques to estimate Ho depend on observations of standard candles, for which systematic effects can be difficult to assess. Conceptually, the weakest link in present estimates is that it is anchored to a distance measurement for a single metal poor galaxy (the LMC). On the other hand, observations of water maser sources populating the accretion disks of active galactic nuclei enable estimation of accurate geometric distances with relatively small systematic errors, which can be used to calibrate the distance scale and Ho rigorously. For the nearby galaxy NGC4258, new distances have been obtained from separate analyses of masers and Cepheids. A comparison in the context of HST Key Project analyses will provide a new, high-accuracy, geometric estimate of Ho. In the long term, an estimate with 1% uncertainty may be achieved directly using a sample of more distant galaxies for which robust ``maser distances'' are available. This is one of three presentations that describe recent advances in the estimation of distance to NGC4258 and ramifications for the extragalactic distance scale and cosmology. This research has been funded in part by NASA grants NAG05-10311 and GO-09810.01-A.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.