A physical upper limit on the HI column density of gas clouds

An intriguing fact about cosmic gas clouds is that they all appear to have neutral (atomic) hydrogen column densities smaller than 10(22) cm(-2). Observations of damped Ly alpha (DLA) absorption systems further indicate that the maximum N-HI decreases with increasing metallicity. It is generally assumed that this trend is due to a dust-induced selection bias: DLA systems with high N-HI and high metallicity contain so much dust that the background quasi-stellar object becomes too dim to be included in optically selected surveys. Here it is argued that this explanation may not be viable. Instead, it is proposed that conversion to molecular hydrogen determines the maximum H I column density. Molecular hydrogen forms on the surface of dust grains and is destroyed by photodissociation. Therefore, the molecular fraction correlates with both the dust content and, because of self-shielding, the total hydrogen column density, and anticorrelates with the intensity of the incident UV radiation. It is shown that the first relation can account for the observed anticorrelation between the maximum N-HI and metallicity.