2019ApJ...877....4L

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). Hα emission in extended halos (analogous to the extended Lyα 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. Hα fluorescence emission from this gas can be used to place strong constraints on the local ultraviolet background and on gas flows around galaxies. Detecting Hα emission from the diffuse intergalactic medium (the "cosmic web") is beyond current capabilities but would be possible with a hypothetical 1000-lens Dragonfly array.