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A Galactic Mystery

Astronomers have learned that the centre of our Milky Way galaxy harbours a long-sought black hole. But the finding has raised even more questions than before.

by Trudy E Bell

In the most suspenseful detective stories, the mystery deepens even as the plot reveals more clues. So has it been in real life for astrophysicists investigating the centre of our Milky Way galaxy. They hoped that NASA's Chandra X-ray Observatory would reveal a long-suspected black hole there - and indeed it did. But Chandra's revelations have raised new questions that baffle scientists perhaps even more than before.

A black hole is an object both so massive and so compact that not even light itself can escape its staggering gravity. For decades, theorists have argued that giant stars (ones at least 10 times as massive as our Sun) routinely end their lives as supernovas - catastrophic explosions that spray matter light-years through interstellar space, leaving behind only a dense remnant of the original star. If the remnant exceeds about 3 solar masses, it will become a black hole.

In 1974, Astronomer Royal Sir Martin Rees proposed that supermassive black holes - ones with a million or even a billion solar masses - might exist within the centres of some galaxies. The galaxies he had in mind have impressively active nuclei (centres) that shine as brightly as 30 billion or more Suns. They glow, unsteadily flickering, at all wavelengths from radio to gamma rays, and they spew powerful jets of charged particles into space. Rees reasoned that black holes gobbling matter were the sources of such turmoil.

"There's no other way we can think of that active galactic nuclei (AGNs) could put out so much energy," says Donald Kniffen, Chandra program scientist at NASA's Office of Space Science at NASA headquarters. "The only accepted theory is black holes." Furthermore, there's a dawning awareness that active galaxies aren't the only ones that might harbour such "monsters in the middle." Ordinary galaxies like the Milky Way have them, too.


A Chandra X-ray Observatory image of the central region of our Milky Way galaxy. A supermassive black hole lurks inside the bright white patch near the centre of the image.

In 1974, even as Rees was speculating about black holes in active galaxies, American radio astronomers Bruce Balick and Robert Brown were observing the relatively quiet centre of our own galaxy. There they discovered a compact and variable radio source that looked much like a faint quasar - a type of far-away AGN that astronomers normally find near the edge of the observable Universe. But this object was "only" 26,000 light-years away, in our own cosmic backyard! Because it appeared to be inside a large, extended radio source already known as Sagittarius A, they named it Sagittarius A* (pronounced "A-star").

X-ray telescopes like the Chandra X-ray Observatory can detect the fiery glow from super-heated gas swirling into a black hole.

Over the next two decades, astrophysicists painstakingly observed Sagittarius A* at radio, optical, and near-infrared wavelengths. The breakneck speed (up to 1400 km/second) of gas and stars swirling around in the centre of the Milky Way began to convince them that something small yet massive - some 2.6 million solar masses - was indeed lurking in our galaxy's centre Was it a supermassive black hole, or just millions of closely packed more-or-less ordinary stars?

Only X-ray observations could provide definitive evidence - both because X-rays are a characteristic final silent scream of matter as it is engulfed forever by a black hole, and only X-rays can penetrate the thick gas and dust obscuring our direct view of the galactic centre Thus, a race was on to be the first to detect X-rays from Sagittarius A*.

Just months after its launch in July, 1999, Chandra succeeded. The Great Observatory had pinpointed a source of X-rays that coincided with Sagittarius A*. Astrophysicists, announcing their findings in January 2000, were ebullient at this observational evidence for a supermassive black hole in the Milky Way's nucleus. Just one problem: the X-rays were only a fifth the intensity that theory predicted. In other words, Sagittarius A* was faint - strange, given that active galactic nuclei are so brilliant.

Whatever could the discrepancy mean?

Follow-up radio and X-ray observations led astronomers to an answer: Ten thousand years ago a supernova exploded very close to Sagittarius A*. The fast-expanding gases swept away much of the local interstellar gas and dust, preventing material from falling into the Milky Way's supermassive black hole, thereby "starving" it. Less material falling into the black hole meant fewer X-rays being emitted.


Chandra X-ray Observatory images of Sagittarius A and A*.

Nevertheless, some material is still infalling. In 2001, right before Chandra's vigilant X-ray eye, Sagittarius A* suddenly brightened. Within minutes it was 45 times its normal intensity. Then it faded back to its pre-flare level about three hours later. The energy released corresponded to the black hole suddenly having engulfed a chunk of material with the mass of a comet or asteroid! Moreover, from the specific way the X-rays brightened and dimmed, astrophysicists calculated that Sagittarius A* was only about 15 million kilometres across-less than a quarter the diameter of the orbit of the planet Mercury around our Sun. This observational evidence of small size coupled with enormous mass seemed to clinch the case for its being a supermassive black hole.

Still, a key mystery remains: Where did the Milky Way's supermassive black hole come from? For that matter, where do any supermassive black holes come from?

"These are excellent questions," Kniffen declared. "Theorists are puzzling over this. One thought is that supermassive black holes formed when galaxies originally formed. Another thought is that a stellar-mass black hole could start accreting matter and eventually grow into a supermassive one. A third possibility is that supermassive black holes grow from clusters of smaller black holes that merge." Or maybe it's something else entirely.

Recently, Chandra may have discovered an important link between stellar-mass black holes and the supermassive type: a 500 solar mass black hole in the nearby irregular galaxy M82. But it is puzzling, too, because the black hole is not centred in M82's nucleus. Will it eventually "sink" to the centre of M82 and grow to become supermassive? No one knows.

And so the mystery continues. At every turn another clue appears, some questions are answered, and new ones take their place. "We're just scratching the surface of this topic," says Kniffen. If only Sherlock Holmes were an astronomer....


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First Science 2014