paper

Imaging the Lyman-alpha Forest

arXiv:astro-ph/9512138 · doi:10.1086/177707

Abstract

We show that it is now possible to image optically thick $\lya$ clouds in fluorescent $\lya$ emission with a relatively long (hr) integration on a large (m) telescope. For a broad range of column densities ($N\gsim 10^{18.5} \cm^{-2}$), the flux of $\lya$ photons from recombination cascades is equal to times the flux of ionizing photons, independent of the geometry of the cloud. Additional $\lya$ 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 $\lya$ emission flux that is $\sim (2/3) \VEVν/ν_0$ times the flux of ionizing photons, where $\VEVν$ is the mean frequency of ionizing background photons and 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 other $\lya$ systems. Such detections would allow one to make a 2-dimensional map of the $\lya$ forest. By taking a series of such spectra, one could map the forest in three dimensions, revealing structure in the high-redshift universe.

Minor numerical change in abstract, no change in text, 23 pages

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