Letter to the Editor

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

Institut d’Astrophysique de Paris, CNRS-SU, UMR7095, 98bis Bd Arago, 75014 Paris, France
e-mail: krogager@iap.fr

Received: 4 November 2020
Accepted: 20 November 2020

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 l_CNM = 12 × 10⁻³. Compared to recent measurements of the incidence of C I-selected absorbers (EW_λ 1560 >  0.4 Å), the value of l_CNM 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 N_H I.