Evidence is emerging that a small galaxy, with a huge central black hole, must have recently collided with the Milky Way, say astronomers
The Milky Way gives the impression of a beautiful, calm field of stars. At the heart of all this is a supermassive black hole that sits, innocently, at the galactic centre.
In reality, however, our galaxy is a maelstrom of havoc on a scale that is hard to comprehend. And now evidence is emerging that the galactic centre has recently been much more active.
For a start, there is the dramatic recent discovery of two giant bubbles of gamma-ray emitting gas that extend some 20 kiloparsecs to the north and south of the galactic centre. Nobody is quite sure what generated the so-called Fermi Bubbles but they have more than a passing resemblance to the kind of jets emitted by active galactic nuclei.
Then there is the strange level of star formation going on near our supermassive black hole. The huge gravity associated with this black hole ought to tear gas clouds apart and that should make star formation tricky. And yet, the galactic centre is home to three massive clouds in which tens of thousands of stars are currently being born.
While we’re talking about stars, we ought to mention the distinct lack of older stars in the centre. That’s certainly fishy since there’s no shortage of them elsewhere in the galaxy.
Finally, there is the unusual fluorescence coming from iron nuclei in a gas cloud close to our supermassive black hole. This is almost certainly an echo of gamma ray bombardment that must have occurred just a couple of hundred years ago.
Clearly, the centre of our galaxy was once much more active, perhaps even in the time since the American War of Independence.
Today, Meagan Lang at Vanderbilt University in Nashville and a few pals say they think they can explain all this activity.
Their idea is that a small satellite galaxy recently smashed into the Milky Way, bringing with it stars, gas and a huge black hole some 10,000 times more massive than the Sun.
Here’s what they say happened. The new black hole spiralled into our own, but not before slinging away most of the older stars from the galactic centre. Much of the new dust was devoured by our own supermassive black hole, causing it to belch gamma ray-emitting jets into intergalactic space. The Fermi Bubbles are the remnants of these. The rest of the dust became a maelstrom of turbulence, perfect conditions for triggering the star formation we see today.
According to Lang and pals, the collision must have begun when the universe was still young. But they conclude that the collision effectively ended when the new black hole eventually combined with our own, an event that probably happened in the very recent past, perhaps just 10 million years ago.
That’s certainly an exciting idea, indeed one that has a touch of Hollywood about it.
But it’s also a theory that’ll need some careful thinking about, as Land and co readily admit: “While the case for a merger of the Milky Way with a satellite galaxy is not ironclad, it is a plausible explanation that naturally accounts for both the old and young stellar distributions and the recent violent past of Sgr A* [the supermassive black hole at the galactic centre].”
With any luck, it’ll generate some interesting debate, something that cosmologists are generally exceedingly good at.
Ref: arxiv.org/abs/1107.2923: Can A Satellite Galaxy Merger Explain The Active Past Of The Galactic Center?