In order to show that there is a black hole in the center of a galaxy, we have to show that so much mass is packed into such a small volume that no normal object, massive stars or star clusters, could explain this (just as we can for the black hole in the Milky Way) We already know from observations that an accretioning black holes is surrounded by a hot accretion disk with gas & dust, which swirls around the black hole before it falls.
If we assume that the energy emitted by quasars is also generated by a hot accretion disk, then, the size of the disk must be given by the time in which the energy of the quasar changes. With quasars, the visible light emission varies on typical time scales from 5-2000 days, which limits the size of the disk to that many light days.
Quasars vary even faster in the X-ray band, so the light propagation time argument says that this higher-energy radiation is generated in an even smaller area. Therefore, the mass around which the accretion disc rotates must be confined to an even smaller space. If the quasar mechanism contains a large mass, then the only astronomical object that can contain a lot of mass in a very small space is a black hole. In some cases, it turns out that X-rays are emitted from an area that is only a few times of the black hole’s event horizon.
The next task is to “weigh” this central mass on the quasar. As far as our own galaxy is concerned, we use observations of the orbits of stars very close to the center of the galaxy and Kepler’s third law to estimate the mass of the central black hole (the Milky Way). In distant galaxies, we cannot measure the orbit of a single star, but we can measure the orbital velocity of the gas in the rotating accretion disk. The Hubble Space Telescope is particularly suitable for this task because it is located above the diffuse layer of the Earth’s atmosphere and can record spectra very close to the bright center of active galaxies. The Doppler effect is used to measure the radial velocity of rotating materials, etc., to determine their moving speed.
One of the first galaxies examined by the Hubble Space Telescope was our long-time favorite, giant elliptical M87. Hubble Space Telescope images show a hot gas disk (10,000 K) orbiting the center of M87. This discovery is very useful for determining the existence of black holes. Astronomers measured the Doppler shift of the spectral line emitted by this gas, determined its rotation speed, and then used Kepler’s third method to use this speed to determine the mass in the disk.
Modern estimates show that a mass of at least 3.5 billion MSun is concentrated in a small region in the center of M87. So much mass in such a small volume of space must be a black hole. A single black hole that swallowed enough material to form 3.5 billion stars like the sun. Few astronomical measurements have led to such an overwhelming result. What a strange environmentthe neighborhood of such a supermassive black hole must.
A disk of dust and gas that surrounds a 300 million M-Sun black hole in the center of an elliptical galaxy (the bright spot in the center is created by the combined light from stars brought together by the gravitational force of the black hole). The mass of the black hole was in turn derived from measurements of the speed of rotation of the disk. The gas in the disk moves at 155 kilometers per second at a distance of only 186 light years from its center. Given the gravitational pull of the mass in the center, we expect the entire disk of dust to be swallowed by the black hole in billions of years.
But do we have to accept black holes as the only explanation for what is at the center of these galaxies? What could we put in such a small-space that is not a huge black hole? The alternative is the stars. But to explain the masses in the centers of galaxies without a black hole, we need to place at least a million stars in a region the size of the solar system. To fit, they should be only 2 star diameters apart. Collisions between stars would happen all the time.And those collisions would lead to star mergers, and very soon the single giant star they form would collapse into a black hole. So there is really no escape: only a black hole can accommodate so much mass in such a small space.
As we have already seen, observations now show that all galaxies with a spherical cluster of stars, be they elliptical galaxies or spiral galaxies with nuclear bulges, harbor one of these huge black holes at their centers. Among them is our neighboring spiral galaxy, the Andromeda Galaxy M31. The masses of these central black holes vary from just under a million to at least 30 billion the Sun mass. Some black holes can be even more massive, but the mass estimates involve great uncertainties and need to be verified. We call these black holes “supermassive” to distinguish them from the much smaller black holes that form when some stars die. So far, the most massive black holes in stars discovered by gravitational waves detected by LIGO have masses of just over 30 solar masses.