Supermassive black holes are now thought to lie at the centers of many if not all galaxies. Under the right conditions, X-ray emission from hot gas very close to the black holes can stimulate water molecules further out to emit maser (i.e., microwave laser-like) emission. The emission lines from these maser regions are so sharp and strong, and the angles measurable by very long baseline interferometry so exquisitely small (~milliarcseconds) that it is possible to measure maser orbital velocities to within a fraction of a parsec of the black hole. Further, these masers are orbiting so rapidly (~1000 km/s) that after a few years it is possible to measure orbital accelerations and even proper motions. Such measurements can be used to obtain very direct "geometric" distances for their host galaxies, free of the systematic and calibration errors that plague the conventional "distance ladder" method of determining extragalactic distances.
About 15 years ago, RG astronomers used the Very Long Baseline Array of the NRAO to obtain the first extragalactic distance using this technique, for the galaxy NGC 4258 (see figure and caption). This 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%.
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. RG astronomers continue to search for more water masers in active galactic nuclei (SAMBA survey) and with NRAO and MPIfR colleagues have undertaken a 5-year project (WMCP) to measure the expansion rate of the universe (the Hubble constant) to a few percent accuracy by obtaining "maser distances" and recessional velocities for a significant number of external galaxies.
Lincoln Greenhill, Jim Moran, Mark Reid,
An artist's conception of the extremely thin accretion disk surrounding the supermassive black hole at the center of the galaxy NGC 4258. RG astronomers used the Very Long Baseline Array to map the structure and 3D dynamics of this disk (Herrnstein et al. 1999, Nature, 400, 539). 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).