LARGE-SCALE STRUCTURE AND GALAXY DISKS AS LYMAN-ALPHA CLOUDS

Large-scale structure in the universe is enriched by two morphological categories, (i) expanding ''clouds'' of bound ''local groups'' of galaxies and (ii) superclusters, as pointed out in the pioneering work of Antoinette and Gerard de Vaucouleurs. Part of the complexity is due to a ''category overlap'' in length scales, e.g., some galaxy pairs have a larger separation than the size of some rich compact groups. This essay includes conjectures on (presently unseen) galaxies, located in the Voids between superclusters, with highly extended gas disks (out to approximately 250 kpc) providing most of the Lyman-alpha ''forest'' absorption systems. The main postulate is a small central peak value N(max) of the mass surface density for the protodisks of these galaxies, which delays not only the recombination of hydrogen but also the phase transition from warm to cold neutral hydrogen. On this model, star formation starts only when the cold phase is reached, but then results in a violent starburst. If conditions are favorable, this burst results in a mild galactic wind (or a galactic fountain) which removes much of the inner gas disk, but leaves most of the outer disk intact. This leads to a prediction: The ratio of the number of ''damped wing'' to the number of forest lines should be much smaller at low redshifts (HST data, excluding absorption from ordinary visible galaxies) than for z greater-than-or-similar-to 2.