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

We study the thermal structure of the neutral atomic (H i) interstellar medium across a wide range of metallicities, from supersolar down to vanishing metallicity, and for varying UV intensities and cosmic-ray (CR) ionization rates. We calculate self-consistently the gas temperature and species abundances (with a special focus on the residual H₂), assuming a thermal and chemical steady state. For solar metallicity, Z′ ≡ 1, we recover the known result that there exists a pressure range over which the gas is multiphased, with the warm (∼10⁴ K, warm neutral medium (WNM)) and cold (∼100 K, cold neutral medium (CNM)) phases coexisting at the same pressure. At a metallicity Z′ ≈ 0.1, the CNM is colder (compared to Z′ = 1) due to the reduced efficiency of photoelectric heating. For Z′ ≲ 0.1, CR 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′ ≲ 0.01, H₂ cooling becomes important, lowering the temperature of the WNM (down to ≈600 K), and smoothing out the multiphase phenomenon. At vanishing metallicities, H₂ 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 H₂ cooling and heating. We discuss potential implications on the star formation rates of galaxies and self-regulation theories.