The MUSE Extremely Deep Field: The cosmic web in emission at high redshift

Vol. 647
3. Cosmology

The MUSE Extremely Deep Field: The cosmic web in emission at high redshift

Galaxies form within large cosmic filaments of gas and dark matter, which are delineated by massive bright galaxies. Smaller galaxies are also believed to gather with the massive galaxies in these filaments, but they are too faint to be observed. With the MUSE Extremely Deep Field, a 140-hour deep MUSE observation in the Hubble Ultra-Deep Field, Bacon et al. have discovered diffuse extended Ly-alpha emission from redshift 3.1 to 4.5, tracing cosmic filaments on scales of several megaparsecs. These structures match the overdensities of individual Ly-alpha emitting galaxies, which are located at the centers of the very extended Ly-alpha diffuse emission. However, 70% of the total Ly-alpha luminosity from these filaments originates outside any individual Ly-alpha emitting galaxy. The origin of this diffuse Ly-alpha emission has remained an open question. The authors postulate that fluorescent Ly-alpha emission powered by the cosmic UV background can only account for a small fraction of it; they conclude that the bulk of this diffuse emission must come from the unresolved Ly-alpha emission of a large population of ultra-low luminosity Ly-alpha emitting galaxies, provided that they are tightly clustered together. If these Ly-alpha emitters are powered by star formation, then their luminosity function needs to extend down to very low star formation rates, below 10^-4 M_sol/yr. These observations provide the first detection of the cosmic web in Ly-alpha emission in typical filamentary environments as well as the first observational clue indicating the existence of a large population of ultra-low luminosity Ly-alpha emitters at high redshift.