How did the biggest galaxy get so big? Do the biggest galaxies grow fat through the slow accretion of intergalactic gas or is it through collisions with other galaxies? New research that mixes observations and simulations leans on the latter idea. The researchers suspect to become more massive, galactic cannibalism is the way forward.
As reported in The Astrophysical Journal, researchers analyzed how the gases in distant galaxies move to establish if the stars in those objects were formed there or came from other galaxies. In distant massive galaxies, they found evidence of chaotic motion, which suggests the stars within had been cannibalized from elsewhere.
“We found that in old massive galaxies – those around 10 billion light-years away from us – things move around in lots of different directions,” lead author Dr Anshu Gupta from Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), said in a statement. “That strongly suggests that many of the stars within them have been acquired from outside. In other words, the big galaxies have been eating the smaller ones.”
Looking further into the universe is like looking back in time, so these galaxies were snacking on smaller companions about 10 billion years ago. The team looked at older (closer) galaxies and it turned out that they were even more disordered. Once they start eating they don’t stop.
“We then had to work out why ‘older’, closer big galaxies were so much more disordered than the ‘younger’, more distant ones,” added second author ASTRO 3D’s Dr Kim-Vy Tran. “The most likely explanation is that in the intervening billions of years the surviving galaxies have grown fat and disorderly through incorporating smaller ones. I think of it as big galaxies having a constant case of the cosmic munchies.”
Mergers are a key phenomenon for galaxy evolution and the simulations conducted using the IllustrisTNG project – a specially designed set of galaxy formation simulations so big it is run simultaneously across some of the world's most powerful supercomputers – showed that the crucial discriminant here is time.
“The modelling showed that younger galaxies have had less time to merge with other ones,” said Dr Gupta. “This gives a strong clue to what happens during an important stage of their evolution.”
Galactic cannibalism (or cosmic munchies) is common among big galaxies. Our own Milky Way has already eaten more than a few of its companions.