Mapping the Cosmic Web with Lyα Emission

We use a high-resolution cosmological simulation to predict the distribution of H I Lyα emission from the lowredshift (z . 0.5) intergalactic medium (IGM). Our simulation can be used to reliably compute the emission from optically thin regions of the IGM but not that of self-shielded gas. We therefore consider several models that bracket the expected emission from self-shielded regions. Most galaxies are surrounded by extended (& 102 kpc) “coronae” of optically thin gas with Lyα surface brightness close to the expected background. Most of these regions contain smaller cores of dense, cool gas. Unless self-shielded gas is able to cool to T < 104.1 K, these cores are much brighter than the background. The Lyα coronae represent “cooling flows” of IGM gas accreting onto galaxies. We also estimate the number of Lyα photons produced through the reprocessing of stellar ionizing radiation in the interstellar medium of galaxies; while this mechanism is responsible for the brightest Lyα emission, it occurs on small physical scales and can be separated using high-resolution observations. In all cases, we find that Lyα emitters are numerous (with a space density ∼ 0.1h3 Mpc−3) and closely trace the filamentary structure of the IGM, providing a new way to map gas inside the cosmic web. Subject headings: cosmology: theory – galaxies: formation – intergalactic medium – diffuse radiation