In a First, Astronomers Directly Image the Cosmic Web

Astronomers have taken an unprecedentedly detailed glimpse of the cosmic web, the faint network of gaseous filaments that feed galaxies with new material like rivers draining into the ocean. The new data could help us better understand how galaxies form and evolve.

Galaxies may appear as islands in the vast black ocean of space, but they are more interconnected than they seem. They tend to cluster together, and often there are long strands of material joining them. It's like galaxies are bright fairy lights strung out on a barely visible cord. That cord can transfer material along it to galaxies and is thought to play a key role in galaxy formation.

Back in 2015, Christopher Martin (Caltech) used the Cosmic Web Imager at the Palomar Observatory in California to image a small section of this cosmic cord. He revealed a long filament funneling gas into a still-growing protogalaxy in the early universe. However, it was only possible to see the filament because it was illuminated by the intense light of a quasar.

Inspired by those findings, Martin and his team set about designing the Keck Cosmic Web Imager at the W. M. Keck Observatory in Hawai‘i. The plan was to probe darker parts of the cosmic web not conveniently illuminated by quasars. It worked. “Before this latest finding, we saw the filamentary structures under the equivalent of a lamppost,” says Martin. “Now we can see them without a lamp.” The team’s findings are published in Nature Astronomy.

To build up a 3D picture of the cosmic web, the team used spectroscopy. They split light from the distant universe into its constituent colours and looked for the 121.5-nanometer-wavelength emission from hydrogen atoms, known as Lyman alpha. The farther away the emission comes from, the more it will have shifted toward the red end of the spectrum, thanks to the expansion of the space.

The result is a forest-like spectrum, each “tree” representing emission from gas located at different points in space and time. “We are basically creating a 3D map of the cosmic web,” Martin says. “We take spectra for every point in an image at a range of wavelengths, and the wavelengths translate to distance.”

However, the light from the cosmic web is very dim and can be easily confused with light sources closer to home, such as from our atmosphere or the solar system. To get around this issue, the team compared two patches of the sky, which they called “A” and “B.”

“The [cosmic web] structures will be at distinct distances in the two directions in the patches, so you can take the background light from image B and subtract it from A, and vice versa, leaving just the structures,” Martin explains.

The result? According to Martin, astronomers now have "a whole new way to study the universe.” Fellow Caltech team member Mateusz Matuszewski is equally enthused. "We are very excited about what this new tool will help us learn about the more distant filaments and the era when the first stars and black holes formed,” he says.

There is potentially more to come. The team has recently installed the second part of the project, known as the Keck Cosmic Reionization Mapper. It will probe longer wavelengths even further toward the red end of the spectrum, and therefore further away in space and further back in time. The efforts reported here focused on Lyman-alpha emissions shifted to the 350- to 560-nanometer range.

The team’s work has even been turned into a piece of music to celebrate the life of astronaut Michael Anderson, who died in the 2003 Space Shuttle Columbia accident. Martin says he “pretended the filaments were giant violin strings,” " translated the filaments’ masses to frequencies based around middle C. The piece can be heard here.