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

We investigate the thermal and ionization history of Lyman alpha cloudsphotoionized by a time--dependent UV background, including non equilibriumeffects. The results show that it is possible to obtain temperatures as low asT~15000 K (or, equivalently, Doppler parameters b~15 km/s) at z=3 for cloudtotal densities n~10^-4 cm^-3, if (i) the reionization epoch occurred atz_i~10, and (ii) the UV background has a factor 70-100 decrease at the HeIIedge. A trend towards smaller b with increasing redshift is present in theredshift interval z=1-5. Higher densities lead to higher values of b andsmaller hydrogen correction factors, n_{HII}/n_{HI}. The correction factors forhelium are also given. For a hydrogen column density N_{HI}=3 10^{14} cm^{-2},cloud sizes are larger than 100 kpc, consistent with recent observations ofquasar pairs. Pressure confined models, instead, yield implausibly low clouddensities at low redshift, and too small sizes at intermediate redshift. Theimplications of the model are confronted with the available observational data.Comment: 6 pages, 4 figures, mnras in pres