Having said yesterday that M87 was the galaxy boasting the largest supermassive black hole (6.6 billion Suns worth of mass) that we're fairly confident of knowing with some accuracy, I went looking after that for what the very largest known is supposed to be, and that search came up with the supermassive black hole at the center of galaxy OJ 287, at the significant distance of 3.5 billion light years from Earth (M87 is a relatively nearby 53 million light years distant). OJ 287's central black hole has been calculated at 18 billion solar masses; and as if just to emphasize how ridiculously massive that is, fluctuations in its signal have been interpreted to be caused by ANOTHER supermassive black hole orbiting it so closely that it is punching through OJ 287's accretion disk as it goes around and around; that little half of this supposed supermassive binary is a *mere* 100 million solar masses (that's 25 times the size of Sagittarius A*, the supermassive black hole at the center of our own Milky Way galaxy), and it is projected to be swallowed by the 18 billion solar masses black hole in the brief time span of just 10,000 years.
The accuracy of this data has been called into question, though, so who knows. But it does make for an interesting mental image to ponder! And if you'd like it dramatically animated for you, it's covered in this video, which, while going a little too heavy on the CG animation, haunting music, dramatic reading, and zooming and rotating in on still photos you could just find by themselves on the Internet--these things indeed seem to be be almost inescapable in astrophysics videos aimed at the general public--is actually rather good, and has some quite interesting simulations, visualizations, and descriptions of the formation of supermassive black holes in the early universe:
And looking through that series, I found they also have a whole episode about A*! And it even has some info I didn't know about before, such as K. G. Jansky's discovery, published to some fanfare in the New York Times in 1933, of a radio source coming from the center of the galaxy, in the direction of the constellation Sagittarius. Jansky discovered the source with his own large antenna, which in effect became the first radio telescope--and this was all done as part of his work for Bell Labs, who wanted to find the sources of static that could interfere with that fascinating new technology, radio broadcasting!
So that was how A* was first detected, although they didn't know what it was at the time. Another thing from the video that I don't think I knew, although it sounds slightly familiar somehow, is a calculation that there are 20,000 black holes within the central three light years of our galaxy; although, given that we have a very difficult time identifying non-supermassive black holes with any degree of certainty, much less those tens of thousands of light years away, and obscured by a lot of gas and dust, I'd sure like to know how someone arrived at that figure.
Anyhow it's probably the best popular science video on A* I've seen. Here it is: