Probing the heating of the neutral atomic interstellar medium in the Dwarf Galaxy Survey through infrared cooling lines

Abstract

Context. Star formation in galaxies is regulated by dynamical and thermal processes. In the Milky Way and star-forming galaxies with similar metallicity, the photoelectric effect on small dust grains usually dominates the heating of the neutral atomic gas, which constitutes the main star-forming gas reservoir. In more metal-poor galaxies, the lower dust-to-gas mass ratio together with the higher occurrence and luminosity of X-ray sources suggest that other heating mechanisms may be at play. Aims. We aim to determine the contribution of the photoelectric effect, photoionization by UV and X-ray photons, and ionization by cosmic rays to the total heating of the neutral gas in a sample of 37 low-metallicity galaxies. In particular, we wish to assess whether X-ray sources can be a significant source of heating. We also attempt to recover the intrinsic X-ray fluxes and compare them with observations when available. Methods. We used the statistical code MULTIGRIS together with a photoionization grid of Cloudy models propagating radiation from stellar clusters and potential X-ray sources to the ionized and neutral gas. This grid includes physical parameters such as metallicity, gas density, ionization parameter, and radiative source properties. We describe a galaxy as a combination of many 1D components linked by a few physical hyperparameters. We used infrared cooling lines as constraints to evaluate the most likely combinations and parameters. Results. We constrained the heating fractions for the main mechanisms for the first time in a low-metallicity galaxy sample. We show that for the higher metallicity galaxies, the photoelectric effect dominates the neutral gas heating. At metallicities below 1 /8 the Milky Way value, cosmic rays and photoionization can become predominant. We computed an observational proxy for the photoelectric effect heating efficiency on polycyclic aromatic hydrocarbons (PAHs) using the total cooling traced by [CI