Truncated photoionized autogravitating clouds at high redshift.

Models of gaseous self-gravitating spheres photoionized and in pressure equilibrium- bounded by an external pressure have been computed. The photoionizing flux is assumed external and is attenuated by the outer layers of the cloud. The ionizing flux at the Lyman limit is assumed to be in the range 10^-22^-10^-21^ erg s^-1^ cm^-2^ sr^-1^ Hz^-1^ and the abundances are varied from Z_sun_/100 to Z_sun_/3. The cloud consists in a dense core which in absence of turbulence has typical characteristic n_H_ > 2 cm^-3^, T_e_ ~ 100 K, r~5 pc, M ~ 150 M_sun_ surrounded by a large halo with density varying with radius from 0.2 to 10^-3^ cm^-3^. The transition between core and halo is triggered by the thermal instability; it is demonstrated that the physical conditions prevailing just beyond the thermal instability, and thus the overall structure of the halo, are independent of the central density in the core. Hydrogen is generally neutral in the central 1 kpc. The radius of the cloud halo is determined by the external pressure and is typically r = 60 and 10 kpc for P_ext_ = 10 and 100 K cm^-3^ respectively; its mass ranges from 2 10^8^ to 5 10^10^ M_sun_. This implies that typical galactic halos associated with normal galaxies should contain several of these clouds. To constrain the model parameters we compare our results with the observed H I column density distribution and equivalent width ratios of z~2 absorption line systems in QSO spectra. It is found that (i) the external pressured P_ext_ must be at least P_ext_~ 100 K cm^-3^, (ii) the external flux must be lower than 10^-21^ and 5 10^-22^ erg cm^-2^Hz^-1^ s^-1^ sr^-1^ at the Lyman limit for a steep spectrum (α = 0.5) with a break at 54.4 eV of a factor ten respectively. This sets upper limits on the UV metagalactic flux. The H I column density distribution can be reproduced between 10^16^ < N(H I)< 6 10^20^ cm^-2^ and the flattening of this distribution at N(H I)~2 10^20^ cm^-2^ is explained by the transition between fully ionized and neutral hydrogen which only occurs at optical depth of a few tens. The damped Lyα systems arise mainly through warm (T~10^4^ K) neutral hydrogen. The models fail to reproduce the large number of damped Lyα systems with column densities larger than about 10^21^ cm^-2^ due to the small size (~5 Pc) of the neutral cold core. Turbulent motions substantially increase the size of these cold cores (~300 pc for T_turb_~4000 K). A possibility to account for these damped Lyα systems could then be the presence of typically 100 of such cores in each cloud of 10 kpc radius. Furthermore, these turbulent motions help stabilizing the large envelope.