OBSERVATIONAL REQUIREMENTS FOR Lyα FOREST TOMOGRAPHIC MAPPING OF LARGE-SCALE STRUCTURE AT z ∼ 2

The z greater than or similar to 2Ly alpha forest traces the underlying dark matter distribution on large scales and, given sufficient sightlines, can be used to create three-dimensional (3D) maps of large-scale structures. We examine the observational requirements to construct such maps and estimate the signal-to-noise as a function of exposure time and sightline density. Sightline densities at z = 2.25 are n(los) approximate to [360, 1200, 3300] deg(-2) at limiting magnitudes of g = [24.0, 24.5, 25.0], resulting in transverse sightline separations of < d(perpendicular to)> approximate to [3.6, 1.9, 1.2] h(-1) Mpc, which roughly sets the reconstruction scale. We simulate these reconstructions using mock spectra with realistic noise properties and find that spectra with S/N approximate to 4 per angstrom can be used to generate maps that clearly trace the underlying dark matter at overdensities of rho/ similar to 1. For the VLT/VIMOS spectrograph, exposure times t(exp) = [4, 6, 10] hr are sufficient for maps with spatial resolution is an element of(3D) = [5.0, 3.2, 2.3] h(-1) Mpc. Assuming similar to 250 h(-1) Mpc is probed along the line of sight, 1 deg(2) of survey area would cover a comoving volume of approximate to 10(6) h(-3) Mpc(3) at < z > similar to 2.3, enabling the efficient mapping of large volumes with 8-10 m telescopes. These maps could be used to study galaxy environments, the topology of large-scale structures at high z, and to detect proto-clusters.