Properties of the Lyman alpha clouds from non-equilibrium photoionization models

We investigate the thermal and ionization histories of Ly alpha clouds photoionized by a time-dependent UV background, including non-equilibrium effects. The results show that it is possible to obtain temperatures as low as T similar to 15000K (or, equivalently, Doppler parameters b similar or equal to 15 km s(-1)) at z = 3 for cloud total densities 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 70-100 decrease at the He II edge. A trend towards smaller b with increasing redshift is present in the redshift interval z = 1-5. Higher densities 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 larger than 100 kpc, consistent with recent observations of quasar pairs. Pressure-confined models, instead, yield implausibly low cloud densities at low redshift, and sizes that are too small at intermediate redshift. The implications of the model are confronted with the available observational data.