Probing [CLC]Lyα[/CLC] Absorbers with Double Lines of Sight

Study of Lyman-alpha absorption lines in the spectra of double line of sight(DLOS) quasars holds the promise of diagnosing the nature of the structuresthat give rise to Lyman-alpha absorption. Based on simulations of DLOS withvarious separations through a single population of absorbers, four tests havebeen designed to diagnose absorber mass distributions (smoothly varying withradius or irregular), geometries (spherical or disk/slab--like), and kinematics(isotropic or systematic velocities). Applying the tests to existing data atredshifts around two we find that: (1) The observed neutral hydrogen columndensity (N(HI)) distributions of Lyman-alpha lines coincident to both LOS areconsistent with a smooth mass distribution. (2) Observed large anticoincidentN(HI) are not consistent with a single population of smooth sphericalabsorbers, which should exhibit sharp cut--offs at small N(HI) in the N(HI)distribution. (3) There is marginal evidence that the observed RMS value ofvelocity differences between coincident lines increases with DLOS separation asis expected for disk/slab--like absorbers that have systematic velocity fields.(4) The observed velocity dispersion along a single LOS is small compared tothe RMS difference between widely separated LOS, which is not consistent withmodels of cloudlets moving isotropically within a spherical structure. Overall,only a smooth disk/slab--like model with systematic velocities remainsconsistent with the inferred properties of a single population of Lyman-alphaabsorbers.