This diagram presents the strong evidence for the existence of a supermassive black hole at the center of galaxy M87. The blue and red tracings represent the spectra of the light from the two locations marked by the blue and red circles on the inset image of the gaseous disk in M87.
The vertical axis of the graph is the intensity of light and the horizontal axis is wavelength. The horizontal displacement on this diagram between the blue and red tracings is due to the Doppler effect. The wavelength of light coming from a source that is approaching the observer is systematically blue-shifted to shorter wavelengths (to the left in this diagram), while light coming from a receding source is red-shifted to longer wavelengths (to the right).
The displacement between the two spectra indicates that, at 60 light years from the center of the galaxy where the observations were made, the gas disk in M87 is turning at the enormous rate of 1.2 million miles per hour. Without the presence of an immense inward gravitational attraction, a disk of gas rotating this rapidly would fly apart. Additional HST observations confirmed that the disk is rotating this rapidly, and that the speed of rotation increases toward the center. The disk is tilted with respect to the line of sight from Earth. As seen here, the disk is turning in the clockwise direction.
The gas disk's huge rotation speed is consistent with the presence of a central black hole of close to 3 billion times the mass of the Sun. Radio astronomy observations have shown previously that the size of the central energy source in M87, at the base of the jet, is a tiny fraction of this 60 light year radius. The combination of enormous mass and very small size implicates a supermassive black hole as the central object of M87. There may be alternate explanations, but they are even more extraordinary and unlikely.
Instrument: Faint Object Spectrograph
Credit: H. Ford (STScl and Johns Hopkins University), and NASA