Faintly glowing wisps of gas, excited by the intense light of surrounding star-forming galaxies, have given astronomers a rare glimpse of one of the Universe’s largest but most elusive features – the intergalactic filaments of the cosmic web. In a new study, researchers report the detection of individual filaments of intergalactic gas spanning among young galaxies in a newly forming cluster – and fueling their growth. “These observations of the faintest, largest structures in the universe are a key to understanding how our Universe evolved through time, how galaxies grow and mature, and how the changing environments around galaxies created what we see around us,” writes Erika Hamden in a related Perspective. The analysis was done on SSA22, a massive proto-cluster of galaxies located about 12 billion light years away in the constellation of Aquarius. Galaxy clusters, the most massive gravitationally bound structures in the Universe, can contain anywhere from hundreds to thousands of galaxies. However, despite their colossal size and the vast amounts of gas and dark matter they contain, it’s predicted that most of the gas in the Universe resides in the spaces in between clusters. Cosmological simulations predict that more than 60% of the hydrogen created during the Big Bang is distributed as long filaments, which thread through the intergalactic medium and form the so-called “cosmic web.” Where these filaments cross, galaxies form and are fed by streams of cooling gas. Most of what is known about the cosmic web is theoretical because direct observations of the faint filaments have remained elusive. Using the Multi Unit Spectroscopic Explorer on the European Southern Observatory’s Very Large Telescope, Hideki Umehata and colleagues detected and mapped the light emitted by hydrogen irradiated by the galaxies within a distant proto-cluster. The results show that the gas is arranged into long filaments, extending over more than one million parsecs, and providing fuel for intense formation of stars and for the growth of super massive black holes within the proto-cluster. This is in accordance with predictions of models of galaxy formation.
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