Spectroscopy of the double quasars Q1343+266A, B: A new determination of the size of Lyman-alpha forest absorbers

We have obtained spectroscopy of Q1343+266A, B, a pair of quasars at redshift z = 2.03 with a projected separation of 9″.5. This system is well suited for probing the Lyα forest, since the two-component spectra show several Lyα lines in common and several others not. Using Bayesian statistics, under the idealization of uniform-radius spherical absorbers, we find that the Lyα cloud radius at z ≈ 1.8 lies in the range 40 h^-1 kpc < R < 280 h^-1 kpc with 98% confidence in a (Ω₀, Λ₀) = (1, 0) model universe, where h ≡ (H₀/100 km s^-1 Mpc^-1). The median value of R is 90 h^-1 kpc. These numbers scale up by a factor 1.44 for (Ω₀, Λ₀) = (0.1, 0) and by a factor 1.85 for (0.1, 0.9). Pressure-confined and freely expanding Lyα cloud models in which the absorbers formed in a significantly more compressed state at z ≫ 2 are contradicted by these new data, as are models involving stably confined gas concentrations in photoionization equilibrium within minihalos of cold dark matter. The comoving density of Lyα forest objects at z ∼ 2 is ∼0.3 h (R/100 kpc)^-2 Mpc^-3 in a (1, 0) model universe. This suggests a possible identification of Lyα clouds with the unvirialized, collapsing progenitors of the faint, blue galaxies at z ∼ 1.