On the Detectability of Visible-wavelength Line Emission from the Local Circumgalactic and Intergalactic Medium

We describe a new approach to studying the intergalactic and circumgalactic medium in the local universe: direct detection through narrowband imaging of ultralow surface brightness visible-wavelength line emission. We use the hydrodynamical cosmological simulation EAGLE to investigate the expected brightness of this emission at low redshift (z 0.2). H emission in extended halos (analogous to the extended Ly halos/blobs detected around galaxies at high redshifts) has a surface brightness of 700 photons cm−2 sr−1 s−1 out to ∼100 kpc. Mock observations show that the Dragonfly Telephoto Array, equipped with state-of-the-art narrowband filters, could directly image these structures in exposure times of ∼10 hr. H fluorescence emission from this gas can be used to place strong constraints on the local ultraviolet background and on gas flows around galaxies. Detecting H emission from the diffuse intergalactic medium (the "cosmic web") is beyond current capabilities but would be possible with a hypothetical 1000-lens Dragonfly array.