HI Column Density Distribution Research Articles

Intergalactic Absorption Due to Extended Gaseous Clouds at High Redshifts

In order to examine the line-blanketing effect due to gaseous clouds at high redshifts, we have calculated the ionization states of gas clouds of subgalactic mass scale exposed to a UV radiation field that can reproduce the ionization states of the Lyman limit systems (LLSs) along the line of sight to quasar HS 1700 + 6416. We also adopted the integrated UV radiation from the observed quasars modified by the intervening absorption due to the Lyman α forest and LLSs as an ionizing spectrum. Our photoionization models can explain the observed HI column-density distribution, the observed CIV/HI ratio and the observed optical depths of HI and HeII. We conclude that the integrated UV background models can well explain the ionization states of both the IGM and Lyman α clouds at z ∼ 2–3. The gaseous mass of the Lyman α clouds follows a power-law form as dn/dMc ∼ Mc-δ with δ ∼ 2–2.5 in the 108 – 1011M⊙ range. The majority of the dark baryon probably consists of the IGM.

Cosmological Conductive/Cooling Fronts as Lyα Forest Clouds

We propose a simple model for the origin and evolution of \lya clouds based on cosmological conductive/cooling fronts. In this model the \lya arises in the interfaces between the IGM and cold clouds that could be tentatively identified with protogalaxies. Most of the properties of the \lya absorbers are reproduced with a very restricted number of assumptions. Among these are the correct range of HI column density, cloud sizes and redshift and HI column density distributions for the absorbers. Several predictions and implications of the model are briefly discussed.

Galactic extinction and Abell clusters

In this paper, we present the results of comparing the angular distribution of Abell clusters with Galactic HI measurements. For most subsamples of clusters considered, their positions on the sky appear to be anti-correlated with respect to the distribution of HI column densities. The statistical significance of these observed anti-correlations is a function of both richness and distance class, with the more distant and/or richest systems having the highest significance (~3 sigma). The lower richness, nearby clusters appear to be randomly distributed compared to the observed Galactic HI column density.