image from wikipedia

Many supermassive black holes, hundreds of thousands to billion times the massive of our sun, have been found at the center of not only our galaxy, but in many others. It seems that this supermassive black hole is a characterizing feature of most galaxies, responsible for holding the group of stars the galaxy together. Although theories of dark matter are still needed to fully describe the gravitational stability of galaxies, this supermassive black hole is responsible for the large density of stars at the center of our galaxy and others.

There are two curious properties of black holes that makes the supermassive black holes unlike typically stellar-size black holes, which are on the order of one solar mass up to 33 solar masses. Any black hole’s size is determined by its Schwarzschild radius, the radius in the gravitational field in which if an object passed it not even light can escape it, and this radius is directly proportional to mass. Because of this property, density and tidal forces can be expressed directly in terms of mass and not mass and radius. The density is equal to the mass divided by the volume of the black hole, which is proportional to the radius cubed, thus the density is proportional to m^-2. Tidal forces, the difference in the strength gravity between successive radii, can be also expressed as proportional to m^-2. What this means for supermassive black holes is that they are not very dense and have very weak tidal forces. If you could stand right above the event horizon of a 10 million solar mass black hole, you would feel similar tidal forces we have on Earth. This means stars can exist close to the black holes without getting ripped apart.

Formation of these gigantic black holes is still a process being researched. Black holes are only made from massive stars collapsing in a supernova explosion that leaves behind a black hole. Because there is an upper limit on the size of these massive stars, there is also an upper limit on the size of the black hole left. Scientists can then predict based off how much gas and matter is left near the black hole, how big it will grow to be based off how much of this matter is accreted inside the black hole. But based on these size calculations alone, we would never expect to find supermassive black holes, unless black holes themselves collided together to form even bigger black holes. It is believed now that supermassive black holes are made from older black holes that over time have collided to form bigger and bigger black holes, which then our galaxy forms around. When two older dwarf galaxies collide, it is very often the case that none of the stars collide or touch, but since black holes are at the center and have a much bigger pull of gravity, it is often the case that they combine. But, when scientists have looked further stellar distances back in time to see primordial dwarf galaxies, they have found that these smaller, ancient galaxies have intermediate size black holes on the order of 1,000 to 10,000 solar masses, which was bigger than expected from theoretical calculations based on accretion rates.  The fact that the black holes are bigger than predicted suggests that black holes are somehow getting bigger faster than just based on the gas accreted into them. But from what we know about these dwarf galaxies, they originated from a very isolated existence, and we wouldn’t expect them to be moving near each other often enough to collide and form these bigger black holes. We know that these galaxies begin to move and collide in the future enough to form the supermassive black holes we see today, but it is unclear how these primordial black holes initially formed.

Sources:

http://www.sciencedaily.com/releases/2014/03/140326170320.htm

http://en.wikipedia.org/wiki/Supermassive_black_hole