Matters of Size

In principle, there is not much limit to black hole size. We expect little ones to evaporate quickly, but nobody has seen any of these. The ones we can measure are mostly big - several solar masses - or really really big - millions or billions of solar masses. We have a pretty good idea how the former form from the death throes of giant stars, but the latter are more mysterious. It's also surprising that we haven't measured any intermediate sized ones - a few hundred to a few thousands of solar masses - until now.

The universe has so many black holes that it’s impossible to count them all. There may be 100 million of these intriguing astral objects in our galaxy alone. Nearly all black holes fall into one of two classes: big or colossal. Astronomers know that black holes ranging from about 10 times to 100 times the mass of our Sun are the remnants of dying stars and that supermassive black holes, more than a million times the mass of the Sun, inhabit the centers of most galaxies.

But scattered across the universe like oases in a desert are a few apparent black holes of a more mysterious type. Ranging from a hundred times to a few hundred thousand times the Sun’s mass, these intermediate-mass black holes are so hard to measure that even their existence is sometimes disputed. Little is known about how they form. And some astronomers question whether they behave like other black holes.

Now a team of astronomers has succeeded in accurately measuring — and thus confirming the existence of — a black hole about 400 times the mass of our Sun in a galaxy 12 million light-years from Earth.

Richard Mushotzky from the University of Maryland (UMD) said the black hole in question is a just-right-sized version of this class of astral objects.

“Objects in this range are the least expected of all black holes,” said Mushotzky. “Astronomers have been asking: Do these objects exist, or do they not exist? What are their properties? Until now, we have not had the data to answer these questions.” While the intermediate-mass black hole that the team studied is not the first one measured, it is the first one so precisely measured, “establishing it as a compelling example of this class of black holes,” said Mushotzky.