Like cosmic spiders, dwarf galaxies have been caught feasting on blobs of gas spread across a hidden web. The very same process seems to have fuelled the birth of stars and the growth of galaxies when the universe was young.
The idea that galaxies grew fat by eating from the cosmic web – a giant mesh beaded with clouds of cold gas – has been around for a while, and recent simulations have increased its popularity. As the gas clouds fall into the gravitational clutches of a galaxy, they spark bursts of star formation, the models show.
But the process has been hard to observe in practice. Most galaxies we see in the local universe, like our own Milky Way, are filled with hot gas that warms up approaching material, preventing it from collapsing into stars. And because clumps of cold gas in intergalactic space don’t emit much light on their own, they’re hard to spot.
The clincher would be to watch a blob of fresh gas triggering a burst of star formation in a small galaxy that isn’t building a lot of stars on its own.
“People have been trying to see if they can find evidence for this and its relationship to lighting up the galaxies,” says Chris Churchill of New Mexico State University in Las Cruces.
Now a team led by Jorge Sanchez Almeida, of the Astrophysics Institute of the Canary Islands, has done just that. “It’s as close to a smoking gun as I’ve seen so far,” Churchill says.
To zero in on the splash of fresh gas into a galaxy, Almeida’s group looked at a set of small, faint galaxies with a low proportion of elements heavier than hydrogen and helium. They were able to infer how oxygen levels varied across these galaxies’ discs.
They found that the bright, star-forming regions had only about a tenth as much oxygen as was found elsewhere in these galaxies. This was a sign of newly arrived gas powering star formation, they concluded: it had to be recently added, because any old gas would lose its distinctive chemical signature within a few hundred million years through being stirred up into a homogenous gloop.
“We are left with just one possibility,” Almeida says. “We’ve picked them up right at the moment that they got fresh gas.”
Blobs of gas moving along the cosmic web could explain the very existence of these galaxies, he says. Perhaps they are just normal dwarf galaxies that have been spurred to make new stars by fresh gas, the recent arrival of which diluted their heavy element content.
The process, if confirmed by more observations, could also explain more. “It glues together several things,” says William Keel at the University of Alabama. Matching star-forming regions to cosmic web threads could let us look at both the composition and the distribution of this nearly invisible gas. “The web is just absurdly difficult to observe,” Keel says.
7 September 2015
By Joshua Sokol
Journal reference: Astrophysical Journal, DOI: 10.1088/2041-8205/810/2/L15