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H_2O submillimeter emission is a powerful diagnostic of the molecularinterstellar medium in a variety of sources, including low- and high-mass starforming 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 H_2O provides.Both radiative pumping due to H_2O absorption of far-infrared photons emittedby dust and collisional excitation in dense shocked gas have been proposed toexplain the H_2O emission. Here we propose two basic diagnostics to distinguishbetween the two mechanisms: 1) in shock excited regions, the ortho-H_2O3_{21}-2_{12} 75um and the para-H_2O 2_{20}-1_{11} 101um rotational lines areexpected to be in emission while, if radiative pumping dominates, bothfar-infrared lines are expected to be in absorption; 2) based on statisticalequilibrium of H_2O level populations, the radiative pumping scenario predictsthat the apparent isotropic net rate of far-infrared absorption in the3_{21}-2_{12} (75um) and 2_{20}-1_{11} (101um) lines should be higher than orequal to the apparent isotropic net rate of submillimeter emission in the3_{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-massstar-forming regions where the H_2O 75um and submillimeter lines have beenobserved with Herschel/PACS and SPIRE, we show that in most (extra)galacticsources the H_2O submillimeter line excitation is dominated by far-infraredpumping, with collisional excitation of the low-excitation levels in some ofthem. Based on this finding, we revisit the interpretation of the correlationbetween the luminosity of the H_2O 988 GHz line and the source luminosity inthe combined galactic and extragalactic sample.

The importance of radiative pumping on the emission of the H_2O submillimeter lines in galaxies