Seeing the Web Connecting Galaxies Across the Universe

One hundred years ago, we didn’t know there was anything outside of our own galaxy, the Milky Way. Now we know that our puny planet Earth, and everything else, is part of a vast structure called the Cosmic Web. Its scale is difficult to comprehend in any concrete way, and the system’s complexity and magnitude brings our most powerful supercomputers to their knees.

Astronomers have known about the Cosmic Web for some time, as they’ve caught glimpses of it. But a new instrument has given us our most complete view of it yet.

In this colossal, convoluted web, galaxies are connected via vast filaments of gas. This intergalactic medium is like a gaseous system of veins and arteries, feeding gas from one region to another. Anything humans do inside this vast network seems inconsequential, but we have one thing going for us: the power to see it and understand it.

The Keck Cosmic Web Imager (KCWI) is an instrument at the Keck Observatory on Maunakea in Hawaii. It was designed by Christopher Martin, the Edward C. Stone Professor of Physics at Caltech, and the Director of Caltech Optical Observatories. Martin is also the lead author of a new paper in Nature Astronomy titled “Extensive Diffuse Lyman-alpha Emission Correlated with Cosmic Structure.

“We chose the name Keck Cosmic Web Imager for our instrument because we were hoping it would directly detect the cosmic web,” said Martin. “I’m very happy it worked out.”

KCWI is a spectrograph, meaning it can measure light’s properties in the specific range of wavelengths it’s built to analyze. In KCWI’s case, it sees light from 350 nm to 560 nm, within the range of human vision. Inside that range, it can see what’s known as Lyman-alpha emissions.

Lyman-alpha emissions are spectral absorption lines from hydrogen. The emissions are created during electron transitions in neutral hydrogen. Since hydrogen is the most ubiquitous and widespread material in the Universe, observing its electron transitions is an effective way of “seeing” the vast cosmic web of hydrogen filaments connecting galaxies.

The Lyman-alpha forest is a critical concept in this research. The Lyman-alpha spectral lines from extremely distant galaxies and quasars don’t reach us in their pristine state. By the time the light reaches us, it’s passed through multiple intervening gas clouds. The clouds have different redshifts, and that affects the light by forming multiple absorption lines, which astronomers call the Lyman-alpha forest. Each vertical line is like a tree trunk in the forest. The Lyman-alpha forest is an important way to probe the cosmic web, and KCWI is built to do it.

By seeing the forest of spectral lines from distant hydrogen, it traces the gaseous filaments of hydrogen that work their way across the vast distances in the Universe.

Hydrogen is the stuff of stars. Stars form inside massive clouds of hydrogen called molecular clouds. These clouds are inside galaxies, but as the new study makes clear, the Universe’s hydrogen is not isolated within galaxies. Instead, vast filaments of cold dark hydrogen gas connect the galaxies and galaxy groups together.

Chris Martin, the lead author of this new research, was part of a team that discovered evidence of this vast hydrogen transport network back in 2015. That research presented evidence for a still-forming galaxy that receives its flow of cold hydrogen gas from an extended filament. While the galaxy itself was rotating, the gas in the filament was moving at a constant velocity, funnelling gas into the galaxy.

For that research, Martin and his colleagues worked with the KCWI’s predecessor, the CWI. It was installed on the Palomar Observatory, and the KCWI is like an improved version of the CWI. In the 2015 discovery, the gas in the funnel was lit up by a quasar and was easier to observe.

But most of the gas in the Universe’s cosmic web is cold and dark. And the KCWI can see this dark gas in places where its predecessor can’t.

“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.”

This figure from the study shows some of the intricate work needed to create the image of the cosmic web. Green circles are known galaxies, and green squares are regions where some specific spectra are extracted. Panels a, c, d, e, and f are different redshifts, denoted by z ranges. Panel b shows how the cosmic web begins to emerge from the KCWI’s data. Image Credit: Martin et al. 2023.

The Cosmic Web doesn’t jump out of the data and present itself. It takes some astrophysical sleuthing to filter it out of the data. Light from the hydrogen can be confused with light from other sources. But Martin devised a way to work through that.

“We look at two different patches of sky, A and B. The filament 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. I ran detailed simulations of this in 2019 to convince myself that this method would work,” he says.

One of the main goals of cosmology and astrophysics is to understand how galaxies form and evolve. The powerful and expensive JWST was built with several core science goals in mind, and understanding galaxy formation and evolution was one of them. That tells you how much emphasis the space science community places on galaxies. These results from the KCWI make an important contribution to the quest by showing how hydrogen moves through the Universe.

But the KCWI’s observations also play into another of space science’s primary goals: understanding dark matter.

Dark matter, of course, is the predominant type of matter in the Universe. Regular matter, called baryonic matter, makes up only a small percentage of the Universe’s matter. Everything we can see and interact with, including our own bodies, is made of regular baryonic matter. But we don’t really understand the Universe if we don’t understand dark matter. The name itself is just a placeholder. Nobody knows what it is.

But by mapping it out, scientists can start to understand what it is. When Martin and his co-researchers imaged the cosmic web, they also imaged the distribution of regular baryonic matter. Conversely, they also imaged dark matter by subtraction.

“The cosmic web delineates the architecture of our universe,” said Martin. “It’s where most of the normal, or baryonic, matter in our galaxy resides and directly traces the location of dark matter.”

The spectral lines that the KCWI sees are stretched into red-shift in varying degrees, depending on their distance. The images of different wavelengths from the instrument can be stacked together, giving depth. This creates a 3D image of the Universe’s distant hydrogen, or basically the cosmic web.

“We are basically creating a 3D map of the cosmic web,” Martin explains. “We take spectra for every point in an image at a range of wavelengths, and the wavelengths translate to distance.”

However, the KCWI has its limitations, and its successor will overcome some of them. Our view of the Cosmic Web is poised to expand and deepen thanks to this new instrument. It’s called the Keck Cosmic Reionization Mapper (KCRM.)

“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.”

Mateusz Matuszewski, senior instrument scientist and study co-author.

The KCRM can see further into the red, meaning it can see more redshifted light. While the KCWI spans a range from 350 nm to 560 nm, the KCRM can see from 530 nm to 1050 nm. The new instrument’s range means our image of the cosmic web is about to improve.

This image shows all of the light in the electromagnetic spectrum, from weak radio waves, through infrared and visible all the way to deadly gamma rays. Image Credit: ESA

The current research created a 3D map of the cosmic web in a region of space between 10 and 12 billion light-years away. The new instrument, the KCRM, will extend the KCMI’s observations. By sensing more powerfully red-shifted light, it can see further into the past.

“With KCRM, the newly deployed red channel of KCWI, we can see even farther into the past,” says senior instrument scientist and co-author Mateusz Matuszewski. “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.”

Evan Gough

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