Astronomers say all of the galaxies in the universe are connected by a vast cosmic web of filaments, but we've never actually seen this supposed network. That's changed, however, thanks to the tumultuous activity of a distant quasar that's illuminating the celestial backdrop.
We already know about these filaments, at least conceptually, because computer simulations tell us they're there. As the universe cooled after the Big Bang, most of its matter (including and especially dark matter) congealed into a network of filaments that spanned the cosmos. Certain points of this web contained more mass than others, eventually resulting in the formation of stars, galaxies, and galactic clusters. So even though the Big Bang happened long ago and its galaxies are now far apart, virtually everything's still connected within this web of vestigial matter.
But these filaments, which exist as rarefied and highly ionized gas, are invisible and have never been seen by astronomers. We've been able to visualize intergalactic gas by detecting its absorption of light from bright background sources, but that hasn't really shown us how this gas is distributed.
Using the Keck Telescope in Hawaii, astronomers recently detected a very large and bright nebula of gas stretching about two million light-years across intergalactic space. The nebula was lit-up like a Christmas tree thanks to a nearby quasar — a type of active galactic nucleus that shoots intense radiation powered by a supermassive black hole at the center of the galaxy.
"This is a very exceptional object: it's huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar," noted researcher Sebastiano Cantalupo in a statement.
The astronomers then took full of advantage of this discovery. They used it to detect the fluorescent glow of hydrogen gas caused by the intense radiation from the quasar.
"This quasar is illuminating diffuse gas on scales well beyond any we've seen before, giving us the first picture of extended gas between galaxies," said astronomer J. Xavier Prochaska. "It provides a terrific insight into the overall structure of our universe."
It's the first time anyone has captured an image of the cosmic web, clearly showing its filamentary structure.
The quasar, UM 287, is about 10 billion light-years away. The filament it's illuminating is two million light-years across, meaning that it must extend from the quasar's host galaxy into intergalactic space.
But while the discovery confirms current conceptions of how the universe formed and evolved, it also shows some gaps in our knowledge. The filament is far more massive than simulations predicted — containing gas that weighs the equivalent of a thousand billion suns.
"We think there may be more gas contained in small dense clumps within the cosmic web than is seen in our models," said Cantalupo. "These observations are challenging our understanding of intergalactic gas and giving us a new laboratory to test and refine our models."
Read the entire study at Nature: "A cosmic web filament revealed in Lyman-α emission around a luminous high-redshift quasar."