Evidence in M84
The following figure illustrates observations taken with the Hubble Space Telescope Imaging Spectrograph
of a region across the center of the
galaxy M84.
Radial Velocity Distributions
The right portion of the figure is a radial velocity distribution across the slice illustrated in the left
portion of the figure, determined by looking at the Doppler shift of light coming from this region. As one
approaches the nucleus (moving downward in the right image) there is a sudden blue shift, indicating rapid
motion of the gas near the nucleus toward us. The Doppler shift indicates that the velocity toward us
reaches as high as 400 km/s at a distance only 26 light years from the center.
Then suddenly the sign of the radial velocity reverses and a redshift
indicating similar velocities away from us is observed, with this red shift decreasing rapidly as
one moves away from the center (toward the bottom of the right diagram).
A Rotating Disk
The most obvious interpretation of these data is that there is a large rotating disk around the nucleus of
M84 that we are seeing in cross section. Above the nucleus in this image the disk matter is moving toward
us at 400 km/s, and below the nucleus the disk matter is moving away at similar velocities. The only
simple explanation is that this is a disk feeding a supermassive black hole in the center of
M84, since no other explanation could easily account for gas
velocities of these magnitudes near the center. Thus, we may take the "S" shape of such velocity
distributions for galactic cores as telltale signs for an spinning
disk around a supermassive black
hole.