Non-equilibrium effects on line-of-sight size estimates of QSO absorption systems

Estimates of the linear extent of heavy-element absorption systems along the line of sight to a QSO often assume that the cloud is photoionized, and that the temperature takes the equilibrium value where photoheating balances line cooling. We show that rather small deviations from this photoionization equilibrium temperature caused by additional heating processes will lead to an overestimate of the neutral hydrogen fraction, and thus to an underestimate of the thickness of the absorber by about two orders of magnitude. Such temperature deviations are indicated both by observations and by numerical simulations. This interpretation reconciles the discrepancy between the rather small extent of heavy-element-absorption systems parallel to the line of sight obtained from a standard photoionization analysis and the much larger transverse size estimates inferred from the observation of common absorption in the spectra of close quasar pairs.