Imaging the Forest of Lyman Limit Systems

We show that it is now possible to image optically thick Lyα clouds in fluorescent Lyα emission with a relatively long (~20 hr) integration on a large (~10 m) telescope. For a broad range of column densities (N ~> 10¹8. 5^ cm^-2^), the flux of Lyα photons from recombination cascades is equal to ~0. 6 times the flux of ionizing photons, independent of the geometry of the cloud. Additional Lyα photons are produced by collisional excitations when these are the cloud's primary cooling mechanism. For typical physical conditions expected in optically thick clouds, these mechanisms together lead to a Lyα emission flux that is ~2/3/ν₀_ times the flux of ionizing photons, where is the mean frequency of ionizing background photons and ν₀_ is the Lyman limit frequency. Hence measurement of the surface brightness from an optically thick cloud (known to exist, e. g. , from a quasar absorption line) yields a direct measure of the energy in the ionizing radiation background. Moreover, in the same long-slit spectrum, one could\ hope to detect emission from ~200 other Lyα systems. Such detections would allow one to make a two-dimensional map of the distribution of Lyα clouds. By taking a series of such spectra, one could map the clouds in three dimensions, revealing the structure in the high-redshift universe.