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

Estimates of the linear extent of heavy-element absorption systems along theline-of-sight to a QSO often assume that the cloud is photoionized and that thetemperature takes the equilibrium value where photo-heating balances linecooling. We show that rather small deviations from this photoionizationequilibrium temperature caused by additional heating processes will lead to anoverestimate of the neutral hydrogen fraction and thus to an underestimate ofthe thickness of the absorber by about two orders of magnitude. Suchtemperature deviations are indicated both by observations and numericalsimulations. This interpretation reconciles the discrepancy between the rathersmall extent of heavy-element-absorption systems parallel to the line-of-sightobtained from a standard photoionization analysis and the much largertransverse sizes estimates inferred from the observation of common absorptionin the spectra of close quasar pairs.Comment: 11 pages, LaTeX , 5 postscript figures included; accepted by MNRA