7 The cosmic web shows off its glow
THE COSMIC WEB is a vast network of filaments of dark matter and normal matter (stars and gas) strung through the universe like a spiderweb. Galaxies lie along these filaments, with larger groups and clusters of galaxies congregating where filaments cross and intersect.
Although it is all around us, this cosmic web is notoriously difficult to observe. Astronomers can only see it when hydrogen gas, either inside or outside galaxies strung along these filaments, glows. But this hydrogen needs something to light it up: either a galaxy’s stars from within, or a bright light source such as a quasar or galaxy cluster shining on it from the outside. Researchers have seen the latter — but these are rare, specific cases, and seeing only unique circumstances limits our understanding of the universe as a whole.
This sparse picture is now beginning to change. A paper published March 18 in Astronomy & Astrophysics highlights a new image of the cosmic web. Taken by staring at a single region of the sky for
140 hours over a period of eight months, the groundbreaking shot was captured with the Multi Unit Spectroscopic Explorer (MUSE) on the European Southern Observatory’s Very Large Telescope in Chile. After a full year of data processing, the resulting image reveals light from hydrogen strung along filaments of the cosmic web as they appeared 10 billion to 12 billion years ago.
In all, the image shows five filaments at different distances, says study coauthor Thibault Garel at the University of Geneva. These filaments are glowing because, embedded within each structure, he says, are thousands of small, faint galaxies. These galaxies are too small to spot individually, but together,
light from hydrogen within them combines to create the filamentary glow seen in the image.
Says Garel, “Our study can be seen as the first detection of the cosmic web in typical filamentary environments” — rather than in the special conditions under which it’s been spotted before. And the myriad small, faint galaxies lighting up the web in the MUSE image had gone previously undetected. Their presence, Garel says, “has strong implications for our understanding of the formation of galaxies in the universe.” Specifically, he says, it implies that all the bright, massive galaxies we easily observe “are just the tip of the iceberg and that many more much smaller objects do exist.”
Now, the team plans to use state-ofthe-art cosmological simulations to learn more about the newly discovered population of small galaxies by trying to recreate the light they produce. Further down the road, Garel adds, the team hopes to repeat or even improve on the image, potentially with a next-generation instrument called BlueMUSE, which could detect the cosmic web over an even larger part of the sky.