QSO absorption line-systems are a natural part of galaxy formation

Abstract

Galaxy formation is modeled here starting from a protogalaxy as a uniform-density gaseous system in a dark halo that starts in uniform expansion, turns around, collapses, and dissipates. The analysis of this simple picture accounts for most of the properties of the QSO absorption-line systems. Two independent arguments are used to show that galaxies turned around at a redshift of two to four. The radius of maximum expansion was roughly 100 kpc. The dynamics of the collapse and the thermal instability and cloud formation that results are examined. The properties of the clouds compare favorably with the observed properties of QSO absorption-line systems. It is shown that there is a simple observational signature that will decide whether the clouds are pressure-confined by an intergalactic medium, gas gravitationally trapped in minihalos, or the products of thermally unstable protogalaxies. 36 references.