Lyman Alpha Absorption in The Cosmic Web

Dick Bond, CITA

Large scale structure at high redshift is described in terms of the
Cosmic Web picture for $\{S,\Lambda,O,H\}$CDM models: how
galactic-scale ``peak-patches'', filaments and membranes create an
interconnected intergalactic medium. The ideas are applied to
Ly$\alpha$ forest simulations of ``shear-field patches'', which use
smooth particle hydrodynamics for the gas and tree$P^3M$ for the
gravitational forces due to dark matter and gas. The response to
changes in power spectrum shape and amplitude, and in cosmological
parameters, is described: with galaxy-scale {\it rms} fluctuations
$\sim 1$ at $z=3$ and a UV choice motivated by proximity effect
observations, the results agree well with HI and HeII column density
data, and $\Omega_b {\rm h}^2$ derived from UV rescaling is in good
agreement with big bang nucleosynthesis estimates. Clouds are brought
back into the Lyman alpha cloud story: the importance of $\sim 30
\kms$ halos along filaments is shown, an aspect missed by lower
resolution simulations. The signal from the intercloud medium is
dominant for $N_{HI} < 10^{14} {\rm cm}^{-2}$, with contributions from
filaments with overdensities $\sim 5 - 10$ and, at lower column
depths, from membranes connecting the filaments with overdensities
$\sim 2$.