Importance of radiative pumping for the excitation of the H2O submillimeter lines in galaxies

Artikel i vetenskaplig tidskrift, 2022

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 3(21) - 2(12) 75 mu m and the para-H2O 2(20) - 1(11) 101 mu m 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 3(21)<- 2(12) (75 mu m) and 2(20)<- 1(11) (101 mu m) lines should be higher than or equal to the apparent isotropic net rate of submillimeter emission in the 3(21)-> 3(12) (1163 GHz) and 2(20)-> 2(11) (1229 GHz) lines, respectively. Applying both criteria to all 16 galaxies and several Galactic high-mass star-forming regions in which the H2O 75 mu m 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.