Cannibalism plays a big part in the life cycle of galaxies, but some rise from the dead as zombies – including one on a collision course with our own
THROUGHOUT cosmic time, galaxies age a little like humans do: they start out small, grow bigger, stronger and more productive, and eventually shrink and run out of energy as they dwindle towards death.
But recent research presented at the American Astronomical Society (AAS) meeting in Seattle last week shows that there is more than one way to kill a galaxy – and they don't always stay dead.
Unlike in humans, cannibalism seems to be the driving force behind galactic life cycles. Gravity pulls smaller galaxies together to form big ones. Then later on large galaxies – including the Milky Way – continue to eat smaller ones that stray into their clutches.
Such mergers influence a galaxy's health and appearance. In their prime of life, galaxies resemble our own Milky Way: grand spirals full of young stars, with more appearing at a healthy rate. But eventually something shuts them off, and they turn into spherical blobs full of old stars.
The question is how this happens. Is it quick, like the explorer who turns to dust after drinking from a fake Holy Grail in Indiana Jones? Or could it happen so slowly that the galaxy itself may not even realise it is dying?
For a long time the most popular suspect in the ageing process was mergers: two gas-rich spiral galaxies collide and form a new one whose stars are scrambled out of their elegant structure. Astronomers call the young spiral galaxies "blue", because young stars tend to be blue in colour, and the old blobby galaxies "red and dead", because old stars appear red.
That leaves a transition zone between blue and red galaxies called the "green valley", in part because green light falls between blue and red in terms of energy (see diagram), although the stars don't actually look green.
"The green valley is a mess," says Daniel McIntosh of the University of Missouri-Kansas City. "It's like a smorgasbord of all kinds of processes. We have a general picture that galaxies are becoming more red and dead, but how?"
We do know that green galaxies have one thing in common: they are running out of gas. One clue to what might be happening comes from observing the black holes that live at their centres. If a black hole is actively gulping material, it can burp out powerful winds and radio jets that violently expel gas out of a galaxy. Years ago, astronomers noticed that galaxies with active black holes also clustered in the green valley.
"So for a few years, it was sort of, ah, well, this proves everything!" says Kevin Schawinski at ETH Zurich in Switzerland, founder of the citizen science project Galaxy Zoo.
But he realised that a galaxy doesn't have to change from a spiral to a blob in order to change its star formation. Using Galaxy Zoo data, he and his colleagues showed that there are two paths through the green valley: the fast switch-off and the slow fade (Monthly Notices of the Royal Astronomical Society, doi.org/x7m).
The galaxies that die quickly probably get a nudge from a central black hole. Schawinski and his colleagues used a radio telescope in the Netherlands to observe four green valley galaxies whose colours indicate that their ages are slightly different, so you can line them up in a time sequence from youngest to oldest.
In their radio images, presented at the AAS meeting, clouds of gas flee the galaxies over about a million years – a pretty short time for a galaxy. One of them also had echoes of jets from the central black hole that aligned perfectly with the ejected gas clouds.
"The black hole is not doing anything any more, it's quiet now, but this is past action by the jets," Schawinski says. While that doesn't prove that the black hole kicked out the gas, it is suggestive.
The slow route across the green valley is more subtle. Sometimes a galaxy can just run out of gas, or enter a mode where its gas is too hot to collapse into new stars. They keep their youthful spiral shapes, but their star formation fades over many billions of years.
"We call these zombie galaxies," Schawinski says. "They're already dead, but they don't know it yet, so they keep going." The Milky Way may even be such a galaxy, although it's hard to tell from inside it. And according to Schawinski, our nearest neighbour galaxy, Andromeda, is probably a zombie.
Last gasp
Some galaxies refuse to conform to either path. In 2011, Katherine Alatalo of the California Institute of Technology and her colleagues found a galaxy called NGC 1266 that looked old in terms of its shape and its colour, but also had a lot of hydrogen and molecular oxygen, which are usually signs of star formation. It also showed signs of gas flowing out from the central black hole.
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