Expansion and Collapse in the Cosmic Web

We study the kinematics of the gaseous cosmic web at high redshift with Lymanalpha forest absorption in multiple QSO sightlines. Using a simple analyticmodel and a cosmological hydrodynamic simulation we constrain the underlyingthree-dimensional distribution of velocities from the observed line-of-sightdistribution of velocity shear across the plane of the sky. The distribution isfound to be in good agreement with the intergalactic medium (IGM) undergoinglarge scale motions dominated by the Hubble flow. Modeling the Lyman alphaclouds analytically and with a hydrodynamics simulation, the average expansionvelocity of the gaseous structures causing the Lyman alpha forest in the lowerredshift (z = 2) sample appears about 20 percent lower than the local Hubbleexpansion velocity. We interpret this as tentative evidence for some cloudsundergoing gravitational collapse. However, the distribution of velocities ishighly skewed, and the majority of clouds at redshifts from 2 to 3.8 expandtypically about 5 - 20 percent faster than the Hubble flow. This behavior isexplained if most absorbers in the column density range typically detectableare expanding filaments that stretch and drain into more massive nodes. We findno evidence for the observed distribution of velocity shear being significantlyinfluenced by processes other than Hubble expansion and gravitationalinstability, like galactic winds. To avoid overly disturbing the IGM, winds maybe old and/or limp by the time we observe them in the Lyman alpha forest, orthey may occupy only an insignificant volume fraction of the IGM. (abridged)Comment: 63 pages, 26 figures, AAS Latex; ApJ, in pres