Most of us would have a hard time imagining 50 billion anything, let alone the 50 billion Suns that would equate to the universe's largest possible black hole. This maximum mass is what astrophysicists at the University of Leicester recently calculated, according to a paper in the Monthly Notices of the Royal Astronomical Society, and it indicates the size at which a black hole would obliterate the galactic structure needed to sustain itself.
That's pretty damn big, even cosmically speaking. But it's also plenty feasible—the largest known supermassive black hole is S5 0014+81 (above) at 40 billion solar-masses.
So, at the center of almost any massive galaxy we'll find a great big bulge of gas and dust along with a supermassive black hole. The gas is what feeds the black hole normally, offering it more and more mass and thus a stronger and stronger gravitational pull. This pull in turn helps the galaxy grow and develop structure. It's a neat symbiosis between light and dark.
"Astronomers generally agree that the centre of almost every galaxy contains a supermassive black hole," writes Leicester astrophysicist Andrew King. "The observed hole masses correlate tightly with large–scale properties of the host galaxy's central bulge. This initially surprising connection arises because the gravitational potential energy released as a black hole grows offers the most efficient way of extracting energy from ordinary matter, and could potentially destabilize a host galaxy's central bulge."
The upper limit estimated by King isn't a true upper limit, but rather one that can be expected for an observable or luminous supermassive black hole. This should be most black holes in fact, as the process of black hole hole growth, in which nearby material is swallowed up by the hole's gravity, gives off quite a bit of light as the infalling gas experiences more and more friction as it approaches the event horizon. These are the bright jets you see above and below the galactic plane in the image at the top of this post.
When a given black hole hits 50 billion solar-masses, we should expect it to terminally destabilize its galaxy's central gaseous bulge, according to King's calculations. At this point, the black hole goes dark and becomes inactive, or relatively so. "Black holes can in principle grow their masses above [50 billion solar-masses] by non–luminous means such as mergers with other holes, but cannot become luminous accretors again," King notes. "They might nevertheless be detectable in other ways, for example through gravitational lensing."
Starved of a reliable food source, black holes thus cannot grow infinitely large, which is interesting just in itself. Even the most extreme events in the universe are the product of an environment and experience feedback relations with that environment. There is no cheating.
The Leicester paper can also be accessed in open-access form at arXiv.