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

Abstract: Star formation in galaxies is regulated by dynamical and thermal processes. The photoelectric effect on small dust grains usually dominates the heating of the star-forming neutral atomic gas reservoir in metal-rich galaxies, while the lower dust-to-gas mass ratio and the higher luminosity of X-ray sources in metal-poor galaxies suggest that other heating mechanisms may be at play. We calculate the relative contributions of the photoelectric effect, photoionization by UV and X-ray photons, and ionization by cosmic rays to the total heating in a sample of 37 nearby galaxies reaching down to 3% the Milky Way metallicity. We use the statistical code MULTIGRIS together with a grid of Cloudy models propagating radiation from stellar clusters and X-ray sources to the ionized and neutral gas, each galaxy being described as a statistical distribution of many 1D components. Infrared cooling lines from the interstellar medium (ISM) are used as constraints to evaluate the most likely distributions and parameters. We show that the photoelectric effect heating dominates in high-metallicity galaxies (>1/18 the Milky Way value) while cosmic rays and especially photoionization from X-rays become predominant in low-metallicity galaxies. Our models predict reasonably well the X-ray source fluxes in the 0.3-8 keV band using indirect ISM tracers, illustrating that the adopted strategy makes it possible to recover the global intrinsic radiation field properties when X-ray observations are unavailable, for instance in early universe galaxies. Finally, we show that the photoelectric effect heating efficiency on PAHs may be recovered through the [CII]+[OI] / PAH observational proxy only if the other heating mechanisms are accounted for (abridged).