The stuff in the universe is not randomly distributed but organized in a structure called the cosmic web. The most massive clusters of galaxies sit in the nodes, while the threads compose fewer galaxies but plenty of intergalactic gas. 

The distribution and interaction of gas filaments are a major driver of galaxy evolution – but studying them is not easy. Astronomers use the light of background sources to gain insight into these structures, and now researchers have reported in Science the emission of filaments around a proto-cluster 12 billion light-years from Earth.

Proto-clusters are the progenitors of galaxy clusters, which sport hundreds if not thousands of galactic members. The target of the observations is SSA22, known for its impressive size, higher-than-average density, and incredible gas structures. More than 60 percent of all the hydrogen in the universe that formed during the Big Bang is predicted to be spread out across galaxies in the intergalactic medium. This hydrogen is believed to be fueling these young galaxies by providing the building blocks of new stars as well as feeding their supermassive black holes. The findings from this latest study appear to confirm that. By spotting the direct emission of these filaments, the team was able to extrapolate their 3D distribution and find they form as an intergalactic network of hydrogen, connecting galaxies to each other that extend for over 3 million light-years.

The observations were possible because the gas in this proto-cluster is irradiated by the galaxies, making it detectable. The emission was captured by MUSE (Multi-Unit Spectroscopic Explorer) on the European Southern Observatory’s Very Large Telescope. The star-forming galaxies within these filaments were studied by the Atacama Large Millimeter/submillimeter Array.

Most of what we know of the cosmic web comes from simulations that provide visualizations of what we think is the math that governs the cosmos. These simulations have successfully predicted several features found in the cosmos. Beyond the simulations, observations seem to indicate that filaments around proto-clusters could be a general feature of the early universe, which means understanding them is key to unravel how galaxies evolve over time.