2002ApJ...574..599M

I present the results of a search for intergalactic hydrogen clouds in voids. Clouds are detected by their H I Lyα absorption lines in the Hubble Space Telescope spectra of low-redshift active galactic nuclei. The parameter with which the environments of clouds are characterized is the tidal field, for this places a lower limit on the cloud mass density that is dynamically stable against disruption. The summing of the tidal fields along these lines of sight is managed by the use of galaxy redshift catalogs. The analytical methodology employed here has been designed to detect gas clouds whose expansion following reionization is restrained by dark matter perturbations. The end products of the analysis of data are the cloud equivalent width distribution functions (EWDFs) of catalogs formed by sorting clouds according to various tidal field upper or lower limits. Cloud EWDFs are steep in voids [dlog(dN/dz)/dlog(W)=S~-1.5±0.2] but flatter in high tidal field zones (S~-1.5±0.6). The most probable cloud Doppler parameters are ∼30 km.s^–1 in voids and ∼60 km.s^–1 in proximity to galaxies. In voids the cumulative line density at low EW (W≃15 mÅ) is essentially equal to that of the mean EWDF, ∼500 per unit redshift. The void filling factor is found to be 0.87≤f_v≤0.94. The void EWDF is remarkably uniform over this volume, with a possible tendency for more massive clouds to be in void centers. The size and nature of the void cloud population suggested by this study is completely unanticipated by the results of published three-dimensional simulations, which predict that most clouds are in filamentary structures around galaxy concentrations and that very few observable absorbers would lie in voids. Strategies for modeling this population are briefly discussed.