PROBING LY-ALPHA ABSORBERS WITH DOUBLE LINES OF SIGHT

Study of Ly alpha absorption lines in the spectra of double line-of-si ght (DLOS) quasars holds the promise of diagnosing the nature of the s tructures that give rise to Ly alpha absorption. Based on simulations of DLOS with various separations through a single population of absorb ers, four tests have been designed to diagnose absorber mass distribut ions (smoothly varying with radius or irregular), geometries (spherica l or disk/slab-like), and kinematics (isotropic or systematic velociti es). Applying the tests to existing data at z similar to 2 we find the following. (1) The observed neutral hydrogen column density (N-H) dis tributions of Ly alpha lines coincident to both LOSs are consistent wi th a smooth mass distribution. (2) Observed large anticoincident N-H v alues are not consistent with a single population of smooth spherical absorbers, which should exhibit sharp cutoffs at small N-H in the N-H distribution. (3) There is marginal evidence that the observed rms val ue of velocity differences between coincident lines increases with DLO S separation, as is expected for disk/slab-like absorbers that have sy stematic velocity fields. (4) The observed velocity dispersion along a single LOS is small compared to the rms difference between widely sep arated LOS, which is not consistent with models of cloudlets moving is otropically within a spherical structure. Overall, only a smooth disk/ slab-like model with systematic velocities remains consistent with the inferred properties of a single population of Ly alpha absorbers.