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Paper: Investigating Dark Energy with Observations of H2O Megamasers
Volume: 395, Frontiers of Astrophysics: A Celebration of NRAO's 50th Anniversary
Page: 103
Authors: Braatz, J.A.; Reid, M.J.; Greenhill, L.J.; Condon, J.J.; Lo, K.Y.; Henkel, C.; Gugliucci, N.E.; Hao, L.
Abstract: As a complement to cosmic microwave background data, a determination of the Hubble constant (H0) to better than ∼ 3% would provide a powerful constraint on the equation of state of dark energy. Water-vapor mega-masers provide perhaps the best opportunity for measuring direct distances to galaxies out to about 200 Mpc. The Megamaser Cosmology Project is pursuing the ambitious goal of making a precise measurement of H0 by measuring maser distances using the techniques pioneered on NGC4258 by Herrnstein et al. (1999). In recent surveys we have identified several megamasers appropriate for distance measurements. To get the distance, two types of observations are required: spectral-line monitoring to measure the acceleration of gas in the disk, and sensitive VLBI imaging to measure the angular size of the disk, measure the rotation curve, and model the radial displacement of systemic maser features. We have recently mapped two of the masers with VLBI, and here we present preliminary results of the maser in UGC3789. The maser disk in UGC3789 was discovered and monitored with the Green Bank Telescope (GBT) and subsequently imaged with the High Sensitivity Array (VLBA + GBT + Effelsberg). While uncertainties are still large, our preliminary analysis gives results in the expected range (distance ∼ 49 Mpc corresponding to H0 ∼ 68 km s−1 Mpc−1). Ongoing observations should reduce the uncertainty in the distance to ∼ 10%.
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