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 narrow-band imaging of
ultra-low surface brightness visible-wavelength line emission. We use the
hydrodynamical cosmological simulation EAGLE to investigate the expected
brightness of this emission at low redshift ($z$ $\lesssim$ 0.2). H$\alpha$
emission in extended halos (analogous to the extended Ly$\alpha$ halos/blobs
detected around galaxies at high redshifts) has a surface brightness of
$\gtrsim700$ photons cm$^{-2}$ sr$^{-1}$ s$^{-1}$ out to $\sim$100 kpc. Mock
observations show that the Dragonfly Telephoto Array, equipped with
state-of-the-art narrow-band filters, could directly image these structures in
exposure times of $\sim$10 hours. H$\alpha$ fluorescence emission from this gas
can be used to place strong constraints on the local ultra-violet background,
and on gas flows around galaxies. Detecting H$\alpha$ emission from the diffuse
intergalactic medium (the "cosmic web") is beyond current capabilities, but
would be possible with a hypothetical 1000-lens Dragonfly array.
