# Thermal Phases of the Neutral Atomic Interstellar Medium -- from Solar Metallicity to Primordial Gas

## TL;DR

This paper investigates how the thermal phases of the neutral atomic interstellar medium change across different metallicities, revealing the disappearance of multiphase structure at very low metallicities and implications for galaxy star formation.

## Contribution

It provides a comprehensive model of the thermal structure of the neutral atomic ISM over a wide metallicity range, including new analytic expressions for phase transition thresholds.

## Key findings

- Multiphase ISM exists at solar metallicity with coexisting warm and cold phases.
- At low metallicity, cosmic-ray heating dominates, shifting phase transition conditions.
- H$_2$ cooling becomes significant at metallicities below 1 extperthousand, smoothing out multiphase phenomena.

## Abstract

We study the thermal structure of the neutral atomic (H {\small I}) interstellar medium across a wide range of metallicities, from supersolar down to vanishing metallicity, and for varying UV intensities and cosmic-ray ionization rates. We calculate self-consistently the gas temperature and species abundances (with a special focus on the residual H$_2$), assuming thermal and chemical steady-state. For solar metallicity, $Z' \equiv 1$, we recover the known result that there exists a pressure range over which the gas is multiphased, with the warm ($\sim 10^4$ K, WNM) and cold ($\sim 100$ K, CNM) phases coexisting at the same pressure. At a metallicity $Z' \approx 0.1$, the CNM is colder (compared to $Z'=1$) due to the reduced efficiency of photoelectric heating. For $Z' \lesssim 0.1$, cosmic-ray ionization becomes the dominant heating mechanism and the WNM-to-CNM transition shifts to ever increasing pressure/density as the metallicity is reduced. For metallicities $Z' \lesssim 0.01$, H$_2$ cooling becomes important, lowering the temperature of the WNM (down to $\approx 600$ K), and smoothing out the multiphase phenomenon. At vanishing metallicities, H$_2$ heating becomes effective and the multiphase phenomenon disappears entirely. We derive analytic expressions for the critical densities for the warm-to-cold phase transition in the different regimes, and the critical metallicities for H$_2$ cooling and heating. We discuss potential implications on the star-formation rates of galaxies and self-regulation theories.