Astronomers peering back in time by studying galaxies so far away that their light has been travelling for more than half the age of the universe have discovered a new class of quasars – and a clue to the ultimate fate of our own galaxy.
Quasars are the most luminous objects in the universe, emitting as much energy as 10 trillion suns, says Allison Kirkpatrick, an astronomer at the University of Kansas, Lawrence, US, and are associated with gigantic black holes at the heart of the largest galaxies.
“I am a black-hole hunter, and quasars are the most massive of these,” she said at a meeting of the American Astronomical Society in St. Louis, Missouri.
Such giant black holes form when two galaxies, each with black holes at its own heart, collide.
At that point, several things happen.
First, the collision stirs up the gas and dust in these galaxies, pushing it toward their centres. Then, the gas and dust start to fall into the black holes, now in the process of merging.
“They begin to feed rapidly on the gas around them,” Kirkpatrick says. “This is the quasar stage. They are almost universally produced by major mergers.”
Feeding black holes produce large amounts of X-rays, but initially, these X-rays are blocked by the gas and dust clouds that have been drawn into the merging galaxies’ centre.
“It obscures the X-rays,” says Kirkpatrick says – but the heated gas and dust isn’t invisible. Instead of X-rays, it emits vast amounts of infrared light that can be detected by earthly astronomers.
“We have a dust-reddened quasar,” she says, “[with] very massive black-hole activity going on, but hidden from view.”
Then, things shift. The powerful radiation being emitted by the quasar overcomes the gravity drawing dust and gas inward toward it, and rapidly blows dust and gas entirely out of the galaxy. “Now we see a luminous blue quasar,” she says.
Red quasars are common, as are blue ones. But nobody before had ever caught one in the act of making the transition, where the gas and dust have been blown out of the inner part of the galaxy, but not yet out of its outer reaches.
To find one, Kirkpatrick’s team examined the 1600 most active known quasars, looking for ones that were blue – indicating that their cores had been swept sufficiently free of gas and dust for us to see the quasar itself – but which also emitted a lot of infrared light, indicating that their outer reaches still contained rings of hot gas and dust.
And, while such objects were rare, Kirkpatrick’s team found 22 of them, all six to 12 billion light years away, including a galaxy that had both a blue quasar and 100 times more dust than our own Milky Way.
That said, it’s a transition phase. “This new population of quasars are rare and short-lived, she says.
They also mark the beginning of these galaxies’ deaths, because without their gas and dust, they can no longer form new stars. “We believe that it is the massive black hole that kills them,” Kirkpatrick says.
And while her study was peering six billion to 12 billion years back in time, it also predicts the future our own Milky Way. “There is a quasar in the Milky Way’s future,” she says.
That’s because our galaxy will eventually collide with the Andromeda galaxy, a galaxy about the size of our own, currently about 2.5 million light years away.
Both galaxies have giant black holes at their centres. Neither black hole is currently doing anything dangerous, but when they collide, Kirkpatrick says, both will light up dramatically.
“That will dominate our night sky,” she says. “They will be incredibly bright. The nice plane of the Milky Way will be dominated by this bright halo [marking the location of the merging black holes].”
Then, all the gas and dust will be blown out of the merged galaxies and star and planet formation will be shut off. Existing planets will probably survive, Kirkpatrick says, “but you won’t get anything new. They’ll just be going around red dwarfs until eventually [their stars] burn out”.
Not that this is anything of immediate concern to humans. The merger won’t occur for another three to four billion years, Kirkpatrick says. “That will be about the same time the sun has turned into a red giant, so we will have other problems to occupy us at the time.”
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