Imaging the Lyman-alpha Forest

We show that it is now possible to image optically thick $\text{lya}$ clouds in fluorescent $\text{lya}$ emission with a relatively long ($\sim 20$hr) integration on a large ($\sim 10$m) telescope. For a broad range of column densities ($N\text{gsim}{10}^{18.5}{\text{cm}}^{-2}$), the flux of $\text{lya}$ photons from recombination cascades is equal to $\sim 0.6$ times the flux of ionizing photons, independent of the geometry of the cloud. Additional $\text{lya}$ 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 $\text{lya}$ emission flux that is $\sim (2/3) \VEV{\nu}/\nu_0$timesthefluxofionizingphotons,where$\VEV{\nu}$ is the mean frequency of ionizing background photons and ${\nu }_0$ 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) gives 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 $\sim 200$ other $\text{lya}$ systems. Such detections would allow one to make a 2-dimensional map of the $\text{lya}$ forest. By taking a series of such spectra, one could map the forest in three dimensions, revealing structure in the high-redshift universe.