Probing Lyα Absorbers in Cosmological Simulations with Double Lines of Sight

We perform a double line of sight (DLOS) analysis of the Lyman-alpha foreststructures that form and evolve in cosmological N-body/hydrodynamicsimulations. Pairs of simulated spectra, extracted from lines of sightseparated by distances from D=12.5kpc up to 800kpc, and a ``control sample'' ofunrelated lines of sight, are analyzed at redshifts 3, 2, and 1. Coincidentline samples are defined for HI column density thresholds of Nco = 10^{12.5},10^{13}, and 10^{14} per square cm. We find that: 1) Under the assumption of asingle structure size, a Bayesian analysis yields sizes that are larger forsmaller Nco, and at fixed Nco the size decreases with decreasing redshift.However, these derived sizes are found to increase with increasing D indicatingthat the assumption of a single structure size is invalid. 2) The columndensities of coincident pairs are highly correlated for small D, withincreasing scatter as D is increased, consistent with structures that have acentrally peaked N(HI) that decreases gradually with radius. 3) The velocitydifference distribution for coincident lines is very narrow for small D, andwidens as D is increased to meet the expectation for chance coincidences inunrelated lines of sight. This behavior is indicative of organized motionwithin the structures. 4) For small D, the distribution of anticoincident linecolumn densities, Nac, falls steeply as Nac increases from the cutoff value,but has a significant tail at large values which is inconsistent with apopulation of spherical absorbers with sharp edges, and consistent with aflattened geometry. The conclusions reached on the basis of the DLOS analysisare validated by an examination of the three-dimensional structures andvelocity flows in the simulation data.