in Galactic Centers
One thing that makes us rather certain that massive black holes can exist in the
center of active galaxies is that there is strong experimental evidence for
enormous amounts of unseen mass in the centers of some
galaxies (whether active or
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) causes a very 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. If we know the distance to the object, then from simple
trigonometry we can determine the spatial extent in which this mass is distributed
from its angular diameter.
Example: The Velocities and Masses in the Center of M87
We may illustrate these ideas using the previously considered example of the
active galaxy M87. The analysis is outlined in the following figure.
Estimate of the central mass in 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 here within a radius of 3.5 parsecs. See the
Source for a detailed
By using Doppler methods to determine the rotational velocity for matter moving
around the central region, we may use basically Kepler's Laws to determine the
amount of mass that must be contained there.
The Case for Supermassive Black Holes
In this case, and in other examples,
one finds that the centers of galaxies often
contain billions of solar masses in very
compact regions. Since there is far less luminosity in such regions than would be
expected if this mass were in the usual distribution of gas, dust, and stars, a
supermassive black hole is considered by most astronomers to be
the simplest explanation that fits the observations.