A Milky Way Galaxy Galactic Mystery

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.

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....