Imaging the Lyman-alpha Forest

TL;DR

This paper demonstrates that it is feasible to image optically thick Lyman-alpha clouds in fluorescent emission using long exposures on large telescopes, enabling direct measurement of the ionizing background and 3D mapping of the Lyman-alpha forest.

Contribution

It introduces a method to image Lyman-alpha clouds in fluorescent emission, allowing direct measurement of the ionizing background and three-dimensional mapping of the Lyman-alpha forest.

Findings

Fluorescent Lyman-alpha emission can be detected with ~20 hr exposures on 10 m telescopes.

The emission flux relates directly to the ionizing photon flux, enabling background measurement.

Potential to map the Lyman-alpha forest in three dimensions through spectral series.

Abstract

We show that it is now possible to image optically thick $\lya$ clouds in fluorescent $\lya$ emission with a relatively long ($\sim 20 $hr) integration on a large ($\sim 10 $m) telescope. For a broad range of column densities ($N\gsim 10^{18.5} \cm^{-2}$), the flux of $\lya$ photons from recombination cascades is equal to $\sim 0.6$ times the flux of ionizing photons, independent of the geometry of the cloud. Additional $\lya$ 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 $\sim (2/3) \VEV{\nu}/\nu_0$ times the flux of ionizing photons, 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 $\lya$ 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.