Observational Requirements for Lyman-alpha Forest Tomographic Mapping of
  Large-Scale Structure at z ~ 2
  The z > 2 Lyman-alpha (Lya) forest traces the underlying dark-matter
distribution on large scales and, given sufficient sightlines, can be used to
create 3D maps of large-scale structure. 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 = [360, 1200,3300] deg^{-2} at limiting magnitudes of g =[24.0,
24.5,25.0], resulting in transverse sightline separations of d_perp = [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 = 4 per angstrom can be used to generate maps that
clearly trace the underlying dark-matter at overdensities of rho/<rho> ~ 1. For
the VLT/VIMOS spectrograph, exposure times t_exp = [4, 6, 10] hrs are
sufficient for maps with spatial resolution epsilon_3d = [5.0, 3.2, 2.3] h^{-1}
Mpc. Assuming ~ 250 h^{-1} Mpc is probed along the line-of-sight, 1 deg^2 of
survey area would cover a comoving volume of ~ 10^6 h^{-3} Mpc^3 at <z>=2.3,
enabling efficient mapping of large volumes with 8-10m telescopes. These maps
could be used to study galaxy environments, detect proto-clusters, and study
the topology of large-scale structure at high-z.
