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 H-2), 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 (similar to 10(4) K, warm neutral medium (WNM)) and cold (similar to 100 K, cold neutral medium (CNM)) phases coexisting at the same pressure. At a metallicity Z' approximate to 0.1, the CNM is colder (compared to Z' = 1) due to the reduced efficiency of photoelectric heating. For Z' less than or similar to 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' less than or similar to 0.01, H-2 cooling becomes important, lowering the temperature of the WNM (down to approximate to 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.