PROPERTIES OF THE LYMAN-ALPHA CLOUDS FROM NONEQUILIBRIUM PHOTOIONIZATION MODELS

We investigate the thermal and ionization histories of Ly alpha clouds photoionized by a time-dependent UV background, including non-equilib rium effects. The results show that it is possible to obtain temperatu res as low as T similar to 15000K (or, equivalently, Doppler parameter s b similar or equal to 15 km s(-1)) at z = 3 for cloud total densitie s n similar to 10(-4) cm(-3), if (i) the re-ionization epoch occurred at z(i) similar to 10, and (ii) the UV background has a factor of 70-1 00 decrease at the He II edge. A trend towards smaller b with increasi ng redshift is present in the redshift interval z = 1-5. Higher densit ies lead to higher values of b and smaller hydrogen correction factors , n(HII)/n(HI). The correction factors for helium are also given. For a hydrogen column density N-HI = 3 x 10(14) cm(-2), cloud sizes are la rger than 100 kpc, consistent with recent observations of quasar pairs . Pressure-confined models, instead, yield implausibly low cloud densi ties at low redshift, and sizes that are too small at intermediate red shift. The implications of the model are confronted with the available observational data.