Spectroscopy of the Double Quasars Q1343+266AB: A New Determination of the Size of Ly Forest Absorbers

We have obtained spectroscopy of Q 1343+266 AB, a pair of quasars at redshift z = 2:03 with a projected separation of 9.5 arcseconds. This system is well-suited for probing the Ly forest, since the two component spectra show several Ly lines in common and several others not. Using Bayesian statistics, under the idealization of uniform-radius spherical absorbers, we nd that the Ly cloud radius at z 1:8 lies in the range 40h 1 kpc < R < 280h 1 kpc with 98% con dence in a ( o ; o ) = (1; 0) model universe, where h (H o =100 km s 1 Mpc 1 ). The median value of R is 90h 1 kpc. These numbers scale up by a factor 1.44 for ( o ; o ) = (0:1; 0); and by a factor 1.85 for (0.1, 0.9). Pressure-con ned and freely-expanding Ly cloud models in which the absorbers formed in a signi cantly more compressed state at z 2 are contradicted by these new data, as are models involving stably-con ned gas concentrations in photoionization equilibrium within minihalos of cold dark matter. The comoving density of Ly forest objects at z 2 is 0:3h (R=100 kpc) 2 Mpc 3 in a (1,0) model universe. This suggests a possible identi cation of Ly clouds with the unvirialized, collapsing progenitors of the faint, blue galaxies at z 1. Subject headings: galaxies: quasars: aborption lines | galaxies: quasars: individual: Q 1343+266