The structure and evolution of Lyman-alpha forest clouds in the minihalo model

Results are presented for one-dimensional numerical hydrodynamics computations of the structure and evolution of Lyα forest clouds gravitationally confined by dark matter minihalos. The clouds are developed from linear perturbations at high redshift and exposed to either a QSO- or galaxy-dominated metagalactic radiation field at moderate redshifts. The baryonic component of the clouds is assumed to be composed of hydrogen and helium in cosmic abundance. While the emphasis is on spherical systems, slab symmetry is also considered. The effects of photoionization heating and losses from radiative atomic processes and from Compton cooling are included. Three zones may be identified in a collapsed cloud: (1) a quasi-hydrostatic core in thermal equilibrium, (2) a nonhydrostatic intermediate zone out of thermal equilibrium, and (3) a cosmological accretion layer joining onto the Hubble expansion. Most of the measured Lyα forest column densities arise in the intermediate zone. The development of the core would result in a flattening in the column density distribution near log N_HI_ ~ 15-16. The cloud diameters corresponding to a column density of 10^14^ cm^-2^ lie in the range 10-60 kpc, while systems with N_HI_ > 10^15^ cm^-2^ have diameters smaller than 10 kpc. Because of a postphotoionization wind, the ratio of central baryon to dark matter density in the clouds generally lies below the cosmic value, although contraction toward hydrostatic equilibrium increases the ratio with time. Systems with circular velocities exceeding V_c_ ~ 50 km s^-1^ result in clouds which contract until they become Jeans unstable and collapse. The critical column density for collapse is 10^17^-10^18^ cm^ -2^. A minimum Doppler parameter of b ~ 25 km s^-1^ is found for absorption lines with column densities in the range 14 < log N_HI_ < 16. The lower limit is independent of initial cloud density, profile, virialization epoch, depth of the potential well, or geometry of the collapse. Allowing for a cutoff in the radiation field shortward of the He II edge, as may occur if QSOs cannot complete the ionization of helium in the intergalactic medium by z < 5, permits Doppler parameters as small as b = 20-25 km s^-1^. Thus the distribution of Doppler parameters may serve as a probe of the spectral shape of the metagalactic radiation field. A mild correlation of Doppler parameter with neutral hydrogen column density is found in several models for systems with log N_HI_ <~13.5, with Doppler parameters occurring as low as b ~> 20 km s^-1^ for log N_HI_~13. No lines with b < 15 km s^-1^ are found in any of the models computed. No evidence is found for significant distortions from Doppler profiles in any of the spherical models for lines with column densities in the range 13.3 < log N_HI_ < 16, suggesting that even supersonic internal velocities may not be detectable in the profiles. Internal velocities may be detected if both the H I and He II Lyα lines may be measured in the clouds. The hydrogen and helium b-values will concentrate on a two-branched curve, a thermal branch, for which the helium-to-hydrogen b-value is 0.5, and a second velocity branch with a ratio near unity arising from the internal cloud motions. By contrast, significant profile distortions are found in slabs of sufficient size prior to collapse and may be visible in existing high-resolution QSO spectra.