Modeling the statistics of the cold neutral medium in absorption-selected high-redshift galaxies

Abstract

We present a statistical model of the selection function of cold neutral gas in high-redshift (z = 2.5) absorption systems. The model is based on the canonical two-phase model of the neutral gas in the interstellar medium and contains only one parameter for which we do not have direct observational priors: namely the central pressure of an L* halo at z = 2.5, P*. Using observations of the fraction of cold gas absorption in strong H I-selected absorbers, we were able to constrain P*. The model simultaneously reproduces the column density distributions of H I and H2, and we derived an expected total incidence of cold gas at z ∼ 2.5 of lCNM = 12 × 10−3. Compared to recent measurements of the incidence of C I-selected absorbers (EWλ 1560 >  0.4 Å), the value of lCNM from our model indicates that only 15% of the total cold gas would lead to strong C I absorption (EW > 0.4 Å). Nevertheless, C I lines are extremely useful probes of the cold gas as they are relatively easy to detect and provide direct constraints on the physical conditions. Lastly, our model self-consistently reproduces the fraction of cold gas absorbers as a function of NH I.