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

Abstract

We present observations of the Ly alpha forest of the close quasar pair Q1343+2640A (z(em) = 2.029) and B (z(em) = 2.031). We detect eight absorption lines of Ly alpha 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 Ly alpha clouds of 40 h(100)(-1) kpc (where h(100) H-0/100 km s(-1) Mpc(-1), q(0) = 0.5) at a redshift z similar or equal to 1.8. The upper -1 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(100)(-1) kpc. Significant velocity and equivalent width variations are seen with an rms velocity difference of similar to 65 km s(-1) between the common absorption lines along the two lines of sight.