Evidence in M87

Let us now give some examples of observational evidence that provide strong support to the central, supermassive black hole hypothesis for the engine driving active galaxies. Let us begin with a radio galaxy that we have already discussed. The left portion of the following Hubble Space Telescope photograph shows the center of the giant elliptical galaxy M87. This galaxy is believed to contain a supermassive black hole at its center. The observations indicate that approximately 3 billion solar masses are concentrated in a region at the galactic core that is only about the size of the Solar System (see the box below). The diagonal line emanating from the nucleus in the left image is the previously discussed jet of high-speed electrons (synchrotron jet) approximately 6500 light years (2 kpc) long that is probably being ejected from the galactic nucleus by the black hole located there. This is what would be expected for matter swirling around the supermassive black hole, with part of it falling forever into the black hole and part of it being ejected in the high-speed jet seen coming from the nucleus of M87.

The right side of the above figure illustrates schematically Doppler shift measurements made on the central region of M87 that suggest rapid rotation of the matter near the center. The measurement was made by studying how the light from the disk is redshifted and blueshifted by the Doppler effect, using the Faint Object Spectrograph aboard the Hubble Space Telescope. Part of the swirling disk spins in Earth's direction and the other side spins away from Earth, thus causing opposite Doppler shifts. The gas on one side of the disk is moving away from Earth at a speed of about 550 kilometers per second (a redshift). The gas on the other side of the disk is approaching the Earth at the same speed (a blueshift). This high velocity suggests a huge gravitational field at the center of M87, far larger than could be accounted for by the visible stars there.

The Virial Theorem and Masses There are some simple ideas concerning gravitational energy in astronomy constituting what is called the Virial Theorem that allow us to estimate the amount of mass contained in a region. The basic idea is that a large concentration of mass (whether we can see it or not) produces a large gravitational field, and that by observing the velocity of objects in that region we can infer the magnitude of the gravitational field and therefore the mass. (Recall our earlier discussion for Doppler broadening of spectral lines in Seyfert 1 galaxies.) If we know the distance to the object, then from simple trigonometry we can determine the spatial extent over which this mass is distributed from its angular diameter. We may illustrate these ideas using the previously considered example of the active galaxy M87. The best fit to the observed velocities for matter swirling around the central region indicates that more than 3 billion solar masses are contained there. This almost certainly rules out any explanation other than a black hole.