| Abstract
| - H 2O 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 H 2O provides. Radiative pumping due to H 2O absorption of far-infrared photons emitted by dust and collisional excitation in dense shocked gas have both been proposed to explain the H 2O emission. Here we propose two basic diagnostics to distinguish between the two mechanisms: First, the ortho-H 2O 3 21 − 2 12 75 μm and the para-H 2O 2 20 − 1 11 101 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 H 2O level populations, that the apparent isotropic net rate of far-infrared absorption in the 3 21 ← 2 12 (75 μm) and 2 20 ← 1 11 (101 μm) 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 H 2O 75 μm and submillimeter lines have been observed with Herschel/PACS and SPIRE, we show that in most (extra)galactic sources, the H 2O 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 H 2O 988 GHz line and the source luminosity in the combined Galactic and extragalactic sample.
|
