Expansion-cooled Lyman-alpha clouds

Recent observations by Pettini et al. indicate that low-H_HI_ Lyα forest lines have small velocity widths b, and that the velocity widths are positively correlated with N_HI_. We show that these observations can be understood as the result of the adiabatic cooling of expanding clouds. Such clouds have ionization abundances roughly consistent with equilibrium in the presence of the background UV radiation but can be far cooler than the equilibrium temperature. The clouds must expand against some confining agent (gravity or external pressure) in order to evolve to lower b; this same agent may operate in forming the clouds initially. Models of static Lyα clouds that are gravitationally confined by dark "minihalos" have difficulty in explaining the observed N_HO_-b correlation, at least in simple versions proposed to date. Other models in which cloud temperatures are determined by photoatomic equilibrium without expansion cooling must have an implausibly broad range of cloud sizes or extremely short lifetimes in order to conform with the new data. We conclude that the results of Pettini et al., if confirmed, constitute a strong argument for expanding cloud models, and we point out observational predictions which follow from the simplest of these models.