Modeling the Statistics of the Cold Neutral Medium in Absorption-selected High-redshift Galaxies

TL;DR

This paper develops a statistical model of cold neutral gas in high-redshift galaxies, constraining physical parameters and reproducing observed gas distributions, providing insights into the incidence and detectability of cold gas via absorption lines.

Contribution

It introduces a one-parameter model based on the two-phase ISM, constrained by observations, to predict cold gas incidence and properties in high-redshift absorption systems.

Findings

The model constrains the central pressure P* of halos at z=2.5.

It predicts an incidence of cold gas l_{CNM} = 12×10^{-3} at z~2.5.

Only about 15% of cold gas causes strong CI absorption.

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 (P*) of an L* halo at z=2.5. Using observations of the fraction of cold gas absorption in strong HI-selected absorbers, we are able to constrain P*. The model simultaneously reproduces the column density distributions of HI and H$_2$, and we derive an expected total incidence of cold gas at z~2.5 of $l_{CNM} = 12\times 10^{-3}$. Compared to recent measurements of the incidence of CI-selected absorbers (EW$_{CI\,1560}$ > 0.4 {\AA}), the value of $l_{CNM}$ from our model indicates that only ~15% of the total cold gas would lead to strong CI absorption (EW > 0.4 {\AA}). Nevertheless, CI 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(HI).