The sketch at left is an artist's conception of an extremely thin accretion disk surrounding a supermassive blackhole. This is very much what we believe lies at the heart of the nucleus of the active galaxy NGC4258. The members of the radio astronomy division of the Harvard-Smithsonian Center for Astrophysics have used the VLBA to map the structure and 3D dynamics of the disk, as traced by the positions, velocities, and accelerations of water masers that lie within an annular region. The inner and outer radii are 0.14 and 0.29 parsecs. The measured rotation curve is Keplerian to better than 1%, indicating that the disk mass is very much smaller than the blackhole mass of 39 million suns. The case of NGC4258 is unusual in the size of the region that has been directly mapped (0.2 parsecs subtends just a few milliarcseconds on the sky and it corresponds to just 50,000 times the radius of the black hole's event horizon) and the accuracy with which physical quantities, such as the blackhole mass, can be estimated.
In the sketch, the maser emission is represented by glints on the disk surface. The radio spectrum of the maser (bottom) exhibits emission at about the velocity of the galaxy (center), red-shifted emission from the receding side of the disk (left), and blue-shifted emission from the approaching side of the disk (right). Sketch by J. Kagaya (Hoshi No Techou) for M. Inoue (NAO).
A global analysis of the disk dynamics provides a distance estimate for NGC4258 of 7.2 Mpc, with a measurement uncertainty of 0.3 Mpc and systematic uncertainty of 0.4 Mpc (Herrnstein et al. 1999, Nature, 400, 539). This is the most accurate distance ever measured for a galaxy.
The "maser distance" to NGC4258 is especially valuable because the galaxy is close enough to measure the light curves of Cepheid variable stars. Cepheids are a common Standard Candle and distance indicator for galaxies within about 30 Megaparsecs. Other distance indicators present in nearby and more distant galaxies, out to a few hundred megaparsecs, are calibrated using Cepheid distances, thus establishing the extragalactic distance scale (Freedman et al. 2001). The distance to NGC4258 is entirely "geometric" and free of systematic errors common to Cepheid measuremnts. It provides a check on the calibration of Cepheids (e.g., Newman et al. 2001), which is important because the latter is anchored by (1) the estimated and somewhat uncertain distance to the Large Magellanic Cloud, and (2) comparisons among stars with rather different "metals" content. (The light curves of Cepheids in different galaxies and different parts of galaxies are not necessarily the same.)
As of 2005, NGC4258 is one of about 70 active galaxies with known water maser sources. Not all active galaxies exhibit visible maser emission because maser emission from accretion disks is beamed chiefly along the approximate equatorial planes and to a small fraction of the Universe. Recent surveys suggest that 10-20% of galaxies within 200 Megaparsecs show emission bright enough to observe with the VLBA and associated instruments (Braatz et al. 2004; Kondratko et al. 2006). Measuring geometric distances to other galaxies is a priority in observational cosmology and the subject of the Water Maser Cosmology Project.