A mosaic image of the starburst galaxy Messier 82.  NASA-ESA/AP

Black Holes? I'll Take a Medium, Please

In one sense, black holes are just ridiculously exotic. Their surface gravity is so powerful that even something as fast as light can't escape (that's why they're black). And what's actually inside a black hole isn't just strange: it's literally indescribable by any known law of physics.

But while they're among the strangest things in the universe, they aren't especially uncommon. Astronomers now know that black holes with the mass of millions or even billions of stars lurk at the cores of most galaxies, including the Milky Way, while much smaller black holes, containing just a few tens of stars' worth of matter, are scattered all over the known universe.

In theory, there's no reason intermediate-size black holes shouldn't exist as well, with masses of a few hundred or a few thousand stars. But so far, despite some tantalizing hints, nobody has definitively found one. That may just have changed, however: a new report in Nature has flagged just such an object in the nearby galaxy Messier 82, which lies about 12 million light-years from Earth in the direction of the Big Dipper.

The black hole in question weighs about 400 times as much as the Sun, and is "just amazing" in the words of co-discoverer Richard Mushotzky, of the University of Maryland. That's true for several reasons; the first is that this object, known as M82 X-1, has been known about for years because it shines brightly in the X-ray part of the electromagnetic spectrum. That marked it from the start as a candidate black hole, since these voracious cosmic vacuum cleaners suck in gas at such a prodigious rate that the infalling matter heats to the kinds of temperatures that generate X-rays.

Astronomers also knew from the brightness of those rays that M82 X-1 was most plausibly a black hole of intermediate mass--somewhere above 100 but less than a thousand solar masses. The problem: while astronomers know how a small black hole forms (it's created when a massive star dies in a supernova explosion), it's not clear how a black hole of more than 50 or so solar masses comes to be.

That put a premium on making sure they truly had the mass right, and lead author Dheeraj Pasham, a Maryland grad student, used a novel technique to figure out what that mass must be. Astronomers have noted that the X-rays from small black holes in the Milky Way pulsate with a characteristic rhythm that is a consequence of general relativity. "It's kind of complicated," Mushotzky says. "You don't really want to know."

The rate of the pulsations depends on the mass of the black hole, and by carefully analyzing observations from NASA's Rossi X-Ray Timing Explorer satellite, Pasham was able to do that calculation with unprecedented precision. "It took a lot of work," says Mushotzky. "It was not easy to do."

But they did it, and Mushotzky says the resulting mass—428 times the mass of the sun, if you're counting—is a reasonably precise figure. "I wouldn't bet my house on it," he says. "But I might bet my car."

If that answer holds up, it could help solve a longstanding mystery of astrophysics. There's no way a multi-million- or billion-solar-mass black hole could form directly. The giants that lie at the cores of galaxies must have built up over time, from small seeds. But if the seeds were only a few tens of Suns in mass, it's hard to see how they could have grown quickly enough to reach full size by just a billion years after the Big Bang--which they nevertheless did.

A black hole like M82 X-1 would have given those giants a head start, however. So it's tantalizing to wonder if this and other objects like it may be leftovers from the earliest days of the cosmos—the potential seeds of giant black holes that somehow failed to sprout, and which are still hanging around in their original form.

If so, they're like living fossils from the earliest period of cosmic history. It's an idea Mushotzky calls "highly speculative at this point." But it's also highly intriguing.

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