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Astronomy and space

Light from the cosmic web maps filaments across millions of light-years

07 Oct 2019
Cosmic filaments
Cosmic connections: this map made by the MUSE instrument at the Very Large Telescope shows gas filaments in blue running horizontally. The white dots embedded within these filaments are very active star forming galaxies that are being fed by the filaments. The galaxies were detected using the Atacama Large Millimeter/submillimeter Array. (Courtesy: RIKEN)

Part of the web of gaseous filaments thought to permeate intergalactic space has been mapped directly for the first time. Two filaments of the web were observed by an international team of astronomers using the Very Large Telescope (VLT) and Keck II telescope. The team’s discovery heralds an important milestone in our understanding of the largest known structures in the universe, paving the way for future studies with even larger telescopes.

First gaining prominence in the late 1990s, the lambda cold dark matter model predicts that over 60% of all baryonic (normal) matter resides within a complex network of hydrogen gas filaments, which spans the entire universe. Galaxies form either when two filaments cross, or if a filament section is particularly dense. After a galaxy is formed, the surrounding filaments feed it with cool gaseous hydrogen.

When filaments are illuminated by ultraviolet light from galaxies, they should emit light by the process of hydrogen fluorescence. This light has been spotted in structures around galaxies, but not further out into intergalactic regions of space.

Sophisticated background subtraction

In this latest study, Hideki Umehata at the RIKEN Cluster for Pioneering Research in Japan and colleagues took a broader approach: observing filaments using the Multi Unit Spectroscopic Explorer (MUSE) on the VLT, and the Keck Cosmic Web Imager on the Keck II telescope. Both instruments are new instalments on their respective telescopes, and can perform spectroscopy over large fields of view. They were also specifically designed to detect the faintest astronomical objects ever predicted, using sophisticated techniques for light background subtraction.

When the results of the two instruments were combined, the Umehata and colleagues could search for filaments throughout the massive proto-cluster SSA22, situated 12 billion light-years away. Across the entire field observed, the astronomers identified hydrogen fluorescence in the brightest parts of the web, allowing them to map the structures of two parallel filaments permeating the cluster. The scale of this observation – around 3 million light-years – exceeds the size of the dark matter halos of even the largest galaxies. This suggested that the structure connects several galaxies, and is likely to be part of an even larger network.

Many questions about the properties of these clouds remain, including how their velocities can be determined, and how their full extents can be mapped. Umehata’s team hopes that in the future, even larger telescopes and longer observation times will be able to detect even fainter and sparser structures, allowing them to produce the first accurate, cosmic-scale maps of the universe’s largest known structures.

The observations are described in Science.

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