Probing Lyα Absorbers in Cosmological Simulations with Double Lines of Sight

We perform a double-line-of-sight (DLOS) analysis of the Lyα forest structures that form and evolve in cosmological N -body/hydrodynamic simulations. Pairs of simulated spectra, extracted from lines of sight separated by distances from D = 12.5 kpc up to D = 800 kpc, and a "control sample" of unrelated lines of sight, are analyzed at redshifts z = 3, 2, and 1. Coincident line samples are defined for H I column density thresholds N co = 10 12.5 , 10 13 , and 10 14 cm -2 . We find the following: (1) Under the assumption of a single structure size, a Bayesian analysis yields sizes that are larger for smaller N co , and at fixed N co the size decreases with decreasing redshift. However, these derived sizes are found to increase with increasing D , indicating that the assumption of a single structure size is invalid. (2) The column densities of coincident pairs are highly correlated for small D , with increasing scatter as D is increased, consistent with structures that have a centrally peaked N H that decreases gradually with radius. (3) The velocity difference distribution for coincident lines is very narrow for small D and widens as D is increased to meet the expectation for chance coincidences in unrelated lines of sight. This behavior is indicative of organized motion within the structures. (4) For small D , the distribution of anticoincident line column densities, N ac , falls steeply as N ac increases from the cutoff value but has a significant tail at large values, which is inconsistent with a population of spherical absorbers with sharp edges and consistent with a flattened geometry. The conclusions reached on the basis of the DLOS analysis are validated by an examination of the three-dimensional structures and velocity flows in the simulation data.