2022A&A...666L...3G

H₂O submillimeter emission is a powerful diagnostic of the molecular interstellar medium in a variety of sources, including low- and high-mass star-forming regions of the Milky Way, and from local to high-redshift galaxies. However, the excitation mechanism of these lines in galaxies has been debated, preventing a basic consensus on the physical information that H₂O provides. Radiative pumping due to H₂O absorption of far-infrared photons emitted by dust and collisional excitation in dense shocked gas have both been proposed to explain the H₂O emission. Here we propose two basic diagnostics to distinguish between the two mechanisms: First, the ortho-H₂O 3₂₁ - 2₁₂ 75 μm and the para-H₂O 2₂₀ - 1₁₁ 101 μm rotational lines in shock-excited regions are expected to be in emission, while when radiative pumping dominates, the two far-infrared lines are expected to be in absorption. Second, the radiative pumping scenario predicts, based on the statistical equilibrium of H₂O level populations, that the apparent isotropic net rate of far-infrared absorption in the 3₂₁ ← 2₁₂ (75 μm) and 2₂₀ ← 1₁₁ (101 μm) lines should be higher than or equal to the apparent isotropic net rate of submillimeter emission in the 3₂₁ → 3₁₂ (1163 GHz) and 2₂₀ → 2₁₁ (1229 GHz) lines, respectively. Applying both criteria to all 16 galaxies and several Galactic high-mass star-forming regions in which the H₂O 75 μm and submillimeter lines have been observed with Herschel/PACS and SPIRE, we show that in most (extra)galactic sources, the H₂O submillimeter line excitation is dominated by far-infrared pumping, combined in some cases with collisional excitation of the lowest energy levels. Based on this finding, we revisit the interpretation of the correlation between the luminosity of the H₂O 988 GHz line and the source luminosity in the combined Galactic and extragalactic sample.