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The goal of the Water Maser Cosmology Project is to obtain a high-accuracy estimate of Ho. This can be accomplished by establishing a sample of anchor galaxies with well-determined geometric distances. These may be used to directly compute Ho from recessional velocities (corrected by cluster infall models, in the case of relatively close by anchors) or to calibrate other distance indicators and techniques, such as the Tully-Fisher relation and Type-Ia Supernova.
The WMCP uses a relatively new distance indicator found in the nuclei of active galaxies (typically type-II objects). Extragalactic water masers (i.e., sources of microwave laser-like emission) originate in the central parsecs of accretion disks. X-ray emissions from very hot material close to supermassive black holes are believed to stimulate water molecules in cooler material at larger radii to emit maser radiation. In turn, time-series spectroscopy of the masers with individual large aperture radio telescopes and emission mapping with Very Long Baseline interferometers can be used to track the rotation of the accretion disks and determine geometric distances. Because the disk geometries can be mapped with high precision and the underlying orbital dynamics are relatively simple, masers have the potential to be a robust distance indicators independent of the systematics and calibrations common to methods that rely on Standard Candles.
The Very Long Baseline Array of the NRAO was used to obtain the first geometric distance, for the galaxy NGC 4258 (Herrnstein et al. 1999). The effort led to discovery of arguably the first direct evidence for a supermassive black hole in any galactic nucleus, as well as the most accurate single extragalactic distance, 7.2 Mpc with a total uncertainty of 7%. At that time, NGC 4258 was the only galaxy for which a distance measurement could be made. Only approximately 20 masers known.
Since the 1990s, the number of known masers has more than doubled, and on the order of ten of the newly discovered sources are candidates at least as promising for distance measurement as NGC 4258. Discovery has been enabled by time intensive surveys of active galactic nuclei, using the largest radio apertures: the Green Bank Telescope in West Virginia (Greenhill et al. 2003; Kondratko et al. 2006), and the Effelsberg telescope in Germany (e.g., Zhang et al. 2006).
Time-series spectroscopy and mapping of these newly discovered maser-disk systems has begun in earnest, accompanied by parallel efforts to expand the list of candidates. Water maser survey sensitivity and survey size are responsible for the large advance in the number of known maser sources. Braatz et al. (1997), Greenhill et al. (2003), and Kondratko et al. (2006) reported sensitivities on the order of 10-20 mJy for on the order of 1000 galaxies. More recent work, e.g., Braatz et al. (2004), has achieved sensitivities on the order of 1-2 mJy (measured over about 1 km s-1) as a matter of course. The WMCP project comprises a five-year joint effort by investigators at the CfA, NRAO, and MPIfR to execute comprehensive surveys of AGN, identify new disk-masers, map sources, and compile time-series spectra for measurement of orbital accelerations. Ultimately, the project lays the ground work for next generation studies with the proposed Square Kilometer Array observatory (c. 2020), which may be expected to be used to discover thousands of maser sources, and if complemented by VLBI stations around the world, hundreds of maser-disks could also be discovered (Greenhill et al. 2004).
Illustration used for thumbnail link to this page is described here. It was created for M. Inoue (NAO) by J. Kagaya (Hoshi No Techou). (Original art)
