Figure 13.3: Evidence for the presence of a supermassive black hole in the galaxy NGC4258. The top panel is an artist's sketch of the molecular accretion disk at the center of this galaxy, which was detected by means of maser emission from trace concentrations of water vapor. Below this is the spectrum of the emission, which shows discrete features corresponding to cloudlets that have different velocities and, hence, different Doppler shifts. The middle picture shows a radio image of the very center of the disk. The small clumps are the images of the maser-emitting water clouds obtained with radio interferometry superposed on a grid representing the unseen portions of the disk. The plot in the lower left shows the radial velocities of the clouds as a function of position along the major axis of the image. The velocities trace a Keplerian profile corresponding to a central mass of 3.5 x 107 M . The deviations from Keplerian behavior are so small (less than 0.3%) that the central mass must lie almost entirely inside the inner edge of the disk at a radius of 0.13 pc. A black hole is the only known object which could have so large a mass compressed into so small a volume. The image in the lower right panel shows radio emission from the jets, which emerge along the rotation axis of the molecular accretion disk (the jets are indicated by the cones in the artist's sketch).