Common Lyman-alpha absorption toward the quasar pair Q1343+2640A, B: Evidence for large and quiescent clouds

We present observations of the Lyα forest of the close quasar pair Q1343+2640A (z_em = 2.029) and B (z_em = 2.031). We detect eight absorption lines of Lyα common to both spectra and not attributable to metalline systems in the redshift range 1.7 < z < 2.1 and four lines which are seen in one spectrum but not the other. At the 9″.5 separation of the two quasars, this implies a firm lower limit on the characteristic size of the Lyα clouds of 40 h^-1₁₀₀ kpc (where h₁₀₀ ≡ H₀/100 km s^-1 Mpc^-1, q₀ = 0.5) at a redshift z ≃ 1.8. The upper limit on the cloud size is much more uncertain owing to the small number of observed lines, but taking the observed fraction of common lines at face value in the context of a simple model, the absorbers are shown to have radii smaller than about 310 h^-1₁₀₀ kpc. Significant velocity and equivalent width variations are seen with an rms velocity difference of ∼65 km s^-1 between the common absorption lines along the two lines of sight.