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We describe a new approach to studying the intergalactic and circumgalacticmedium in the local Universe: direct detection through narrow-band imaging ofultra-low surface brightness visible-wavelength line emission. We use thehydrodynamical cosmological simulation EAGLE to investigate the expectedbrightness of this emission at low redshift ($z$ $\lesssim$ 0.2). H$\alpha$emission in extended halos (analogous to the extended Ly$\alpha$ halos/blobsdetected around galaxies at high redshifts) has a surface brightness of$\gtrsim700$ photons cm$^{-2}$ sr$^{-1}$ s$^{-1}$ out to $\sim$100 kpc. Mockobservations show that the Dragonfly Telephoto Array, equipped withstate-of-the-art narrow-band filters, could directly image these structures inexposure times of $\sim$10 hours. H$\alpha$ fluorescence emission from this gascan be used to place strong constraints on the local ultra-violet background,and on gas flows around galaxies. Detecting H$\alpha$ emission from the diffuseintergalactic medium (the "cosmic web") is beyond current capabilities, butwould be possible with a hypothetical 1000-lens Dragonfly array.

On the detectability of visible-wavelength line emission from the local circumgalactic and intergalactic medium