2023NatAs...7.1390M

The intergalactic medium represents the dominant reservoir of baryons at high redshift, traces the architecture of the cosmic web dominated by dark matter and fuels ongoing galaxy evolution. The intergalactic medium has been studied using the absorption lines of quasi-stellar objects, including the Lyman-α forest, but these absorption lines are unable to provide the information that emission maps would give. However, because of the low surface brightness and extended, diffuse distribution, direct detection of an emission equivalent to the absorption Lyman-α forest has not been possible with existing instrumentation and observational approaches. Using a purpose-built instrument, with nod-and-shuffle and dual-field subtraction, we have detected an emission Lyman-α forest. The emission forest is highly extended, shows filamentary morphology with filaments connecting galaxies, exhibits statistics like the absorption Lyman-α forest, displays spectra resembling the absorption forest and is correlated with galaxy-traced overdensities consistent with bias like dark matter. We conclude that the emission Lyman-α forest may provide a new tool for tracing a substantial fraction of the cosmic web of baryons and dark matter.