The tangled web we weave

Large-scale simulations of the Universe show a characteristic network of filaments and voids, which is in spectacular agreement with real observations of the distribution of galaxies. Even within the voids, what few galaxies are present are distributed in thin tendrils. Galaxies are like flies caught in this cosmic web of dark matter, except fortunately there aren't any giant spiders coming along to eat them, which is a shame because that would inspire some pretty bad-ass mythology.

But what's in between the galaxies, in the filaments themselves ? They appear, as predicted, to have their own dark matter, acting as a sort of scaffold onto which other material can accrete. Detecting this infalling gas would be pretty neat as this could give clues to galaxy formation and survival. For example, the Milky Way is currently forming stars at such a rate that it ought to run out of gas pretty quickly, so unless we happen to be witnessing it in its final phase of star formation, it's likely that it's being re-supplied from somewhere.

Claims for direct detection of the gas in filaments are many and various, the difficulty being that it's hard to distinguish between material present in the primordial filament and stuff that's been chucked out of galaxies during interactions (not to mention that the gas is especially thin and hard to detect at all). There have been a few credible possibilities though, such as this single giant filament seen by Planck, and a convincing statistical detection from stacking observations of many pairs of galaxies. But information is very, very scant.

This paper takes things to a new level with a direct detection of faint UV emission in a complex of very bright, very distant galaxies. Like the earlier Planck observations, the emission is too extended to likely result from galaxy interactions (it's about 1 Mpc across), but unlike Planck it shows several different structures, and looks a lot more web-like.

The detection here was possible only due to an exceptional circumstance. At a redshift of 3, the Universe was only about 2 billion years old, and therefore a lot smaller and denser than it is today. Star formation activity and supermassive black holes were also churning out energy like nobody's business, but this particular target region is exceptional even by the standards of the time : it has a density of active galaxies about 1,000 times greater than the average of the day. Which is slightly insane and terrifying, but does explain how the web can be detected here when it's normally so faint.

Could it be that this is just unusually extended gas from tidal interactions, and not related to the primordial material in the web ? Probably not. Although a few extensions of similar length are known, the gas here doesn't show much variation in velocity, unlike tidal cases. And its velocity dispersion is lower in the filaments than close to the galaxies, which is what you'd expect if it was cool gas accreting onto the dark matter skeleton. So while there are various other previous claims for detecting parts of the web directly, and a couple of good cases of directing it statistically, this seems like a pretty solid claim on being the first direct detection of an actual proper web and not just a mere bridge between a couple of galaxies.

Gas filaments of the cosmic web located around active galaxies in a proto-cluster

Cosmological simulations predict the Universe contains a network of intergalactic gas filaments, within which galaxies form and evolve. However, the faintness of any emission from these filaments has limited tests of this prediction. We report the detection of rest-frame ultraviolet Lyman-alpha radiation from multiple filaments extending more than one megaparsec between galaxies within the SSA 22 proto-cluster at a redshift of 3.1.