THE SIZE AND NATURE OF LYMAN-ALPHA FOREST CLOUDS PROBED BY QSO PAIRS AND GROUPS

Closely separated QSO pairs and groups make it possible to probe the s ize, geometry, and spatial clustering of Ly alpha forest clouds. Recen t spectroscopic observations of Q1343 + 2640A/B give evidence that the transverse sizes of Ly alpha clouds are very large at redshifts appro ximate to 2 (as reported by Bechtold et al. in 1994). In this paper, w e describe a robust Bayesian statistical method for determining cloud sizes in spherical and in thin disk geometries, apply this method to t he available data, and discuss implications of our results for models of Ly alpha clouds. Under the assumption of a population of uniform-si zed and unclustered Ly alpha clouds, the data from Q1343 + 2640A/B giv e a 99% confidence lower and upper bounds 61 < R < 533 h(-1) kpc on th e radius of spherical clouds at z approximate to 1.8, with a median va lue of 149 h(-1) kpc [(Omega(0), Lambda(0)) = (1, 0), and h = H-0/100 km s(-1) Mpc(-1)]. The baryonic mass of such large clouds, if they are roughly homogeneous and quasi- spherical, is comparable to the baryon ic mass of dwarf irregular galaxies. Their cosmic overdensity is close to the turnaround density but generally below the virialization densi ty, which suggests a population of gravitationally bound but unviriali zed protogalactic objects at z approximate to 2. The comoving volume d ensity of these clouds is similar to that of the faint blue galaxies ( FBGs) at the limiting magnitude B approximate to 26-27, if these FBGs are distributed approximately over the range of redshift from 0.8 to 2 . The timescale for dynamical collapse of overdensities like these clo uds is also comparable to the cosmic time difference between z approxi mate to 2 and z approximate to 1. Both populations of objects show sim ilar weak clustering in space. All this evidence suggests a possible i dentification of Ly alpha clouds as the collapsing progenitors of the FBGs at z similar to 1. We also investigate the other closely separate d QSO pairs with published high-quality spectra: Q0307 - 1931/0307 - 1 932, Q0107 - 0232/0107 - 0235, and the triplet of Q1623 + 268. Imposin g a uniform W-0 greater than or equal to 0.4 Angstrom counting thresho ld on all the line lists, we find a trend of larger inferred cloud rad ius with larger proper separation of QSO pairs, significant at the 3.4 sigma level. This indicates that the idealization of unclustered, uni form-sized spherical clouds does not accurately describe the Ly alpha cloud population. Present data are insufficient to resolve with confid ence whether this effect is due to clustering, filamentary shape, or n onuniform cloud size. There is a suggestion, however, that at low reds hifts a residual population of larger clouds remains.