Self-shielding effects on the column density distribution of damped Lyα systems

We calculate the column density distribution of damped Lyalpha systems, modeled as spherical isothermal gaseous halos ionized by the external cosmic background. The effects of self-shielding introduce a hump in this distribution, at a column density of N-HI similar to 1.6 x 10(17) X-1 cm(-2), where X is the neutral fraction at the radius where self-shielding starts being important. The most recent compilation of the column density distribution, by Storrie-Lombardi & Wolfe, shows marginal evidence for the detection of this feature owing to self-shielding, suggesting a value of X similar or equal to 10(-3). Assuming a photoionization rate of Gamma similar or equal to 10(-12) s(-1) from the external ionizing background, the radius where self-shielding occurs is inferred to be similar to3.8 kpc. If damped Lyalpha systems consist of a clumpy medium, this should be interpreted as the typical size of the gas clumps in the region where they become self-shielding. Clumps of this size with typical column densities of N-H similar to 3 x 10(20) cm(-2) would be gravitationally confined at the characteristic photoionization temperature of 10 4 K if they do not contain dark matter. Since this size is similar to the overall radius of damped Lyalpha systems in cold dark matter models, where all halos are assumed to contain similar gas clouds producing damped absorbers, this suggests that the gas in damped absorbers is in fact not highly clumped.