What happens when a galaxy runs out of fuel to make new stars? It finds that fuel elsewhere, and sometimes, that means resorting to cannibalism.
Yep, cannibalism. A new paper from a team of Australian astronomers published today in the Monthly Notices of the Royal Astronomical Society confirms that when push comes to shove, large galaxies eat smaller ones in order to continue expanding.
Aaron Robotham of the University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR) led a team of researchers who looked at more than 22,000 galaxies. They found that smaller galaxies in our universe are adept at making their own stars; they have plenty of gas on hand to compress into new stars. But bigger galaxies don’t have extra material at their disposal with which to make new stars.
Star formation slows in really massive galaxies because of extreme feedback in the active galactic nucleus, the center of the galaxy that emits large amounts of radiation.
“The topic is much debated, but a popular mechanism is where the active galactic nucleus basically cooks the gas and prevents it from cooling down to form stars,” Robotham said in a statement.
And yet, big galaxies can still grow.
Turns out, galactic survival is sort of like a weird version of survival of the fittest. All galaxies start out small, growing as they collect gas that they efficiently turn into new stars. But not all galaxies grow to a ripe old age. The team found that smaller ‘dwarf’ galaxies are sometimes consumed by their nearby larger counterparts.
Every now and then, a dwarf galaxy is “completely cannibalized by a much larger galaxy,” Robotham said. And in this instance size does matter. The larger galaxies have more gravity and can more readily tear apart their smaller neighbours.
Technically, Andromeda will eat us
Our own Milky Way is sort of on the edge of becoming a cannibal, which is slightly better than being at risk of being cannibalized. Our galaxy has reached a point in its evolution where any growth we see will come from it eating smaller galaxies, not by collecting gas to turn into new stars.
“The Milky Way hasn’t merged with another large galaxy for a long time but you can still see remnants of all the old galaxies we’ve cannibalized,” Robotham said. And we can see its next two meals. Robotham added that the Milky Way is likely to “eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years.”
Unfortunately, the Milky Way will eventually get its comeuppance. In about five billion years, it will merge with the Andromeda Galaxy. The two galaxies will pull each other apart and turn into something new, so the familiar Milky Way we call home will cease to be.
“Technically, Andromeda will eat us because it’s the more massive one,” Robotham said.
Fortunately, we’re unlikely to be around to see the carnage.
Ultimately, galactic cannibalism comes down to gravity. Over billions of years, gravity will cause all the galaxies currently bound together in groups and clusters to merge into a few super-giant galaxies. Along the way, a bigger one is bound to consume a smaller one.
“If you waited a really, really, really long time that would eventually happen but by really long I mean many times the age of the Universe so far,” Robotham said. So neat as it might be, it’s probably not worth holding your breath to see just what happens with the Milky Way gets cannibalized.