The largest, most massive black hole has just been discovered and reported in Nature magazine. It weighs in at a billion times the mass of the Sun. This is interesting as the mass of the black hole at the center of our own Milky Way is a ‘lightweight’ by comparison at only about 4 million (not billion) solar masses.
We know black holes can form as a result of certain types of explosions by huge stars with minimum masses of at least eight times the mass of the Sun. The black holes that results from such explosions, the “stellar mass” black holes, are about 10 solar masses each. They move along with the other stars in the galaxy disk. Then, by a kind of friction, these black holes tend to settle in the center of the galaxy in which they are formed.
This process of shuffling stellar mass black holes to their centers is easy to say, but takes a billions of years. We are not quite sure what happens next, but we think that all these black hole neighbors will coalesce to form one giant object called a ‘supermassive’ black hole.
By this reckoning, a supermassive black hole with a mass as large as 1 billion solar masses should therefore be very old, because it takes such a very long time to build up the mass in the center. Therefore, it came as a complete surprise when this new supermassive black hole was announced which is not an elder or even a child but rather more like a big baby.
The only way one can imagine out of this conundrum is if there was a different, as yet undiscovered route to forming a black hole of a mass in between that of a stellar mass and a supermassive black hole. Some think it is possible for such “seed” black holes to form if there was a mechanism allowing stars in the very early universe to be far more massive than they are today, so that their explosions are larger as is the size of their leftover stellar mass black hole.
Large surveys of galaxies have made this discovery possible. New galaxy surveys are underway which should lead to additional giant baby black holes if they are out there.