Imaging the forest of Lyman limit systems

We show that it is now possible to image optically thick Ly alpha clouds in fluorescent Ly alpha emission with a relatively long (similar to 20 hr) integration on a large (similar to 10 m) telescope. For a broad range of column densities (N greater than or similar to 10(18.5) Cm-2), the flux of Ly alpha photons from recombination cascades is equal to similar to 0.6 times the flux of ionizing photons, independent of the geometry of the cloud. Additional Ly alpha 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 Ly alpha emission flux that is similar to 2/3<v>/v(0), times the flux of ionizing photons, where [v] is the mean frequency of ionizing background photons and v(0) 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 similar to 200 other Ly alpha systems. Such detections would allow one to make a two-dimensional map of the distribution of Ly alpha clouds. By taking a series of such spectra, one could map the clouds in three dimensions, revealing the structure in the high-redshift universe.