2013ApJ...771L..24Y

We report the first systematic study of the submillimeter water vapor rotational emission lines in infrared (IR) galaxies based on the Fourier Transform Spectrometer (FTS) data of Herschel SPIRE. Among the 176 galaxies with publicly available FTS data, 45 have at least one H₂ O emission line detected. The H₂ O line luminosities range from ∼1x10⁵ L_☉ to ∼5x10⁷ L_☉ while the total IR luminosities (L_IR) have a similar spread (∼1-300x10¹⁰ L_☉). In addition, emission lines of H₂O⁺ and H₂¹⁸O are also detected. H₂ O is found, for most galaxies, to be the strongest molecular emitter after CO in FTS spectra. The luminosity of the five most important H₂ O lines is near-linearly correlated with L _IR, regardless of whether or not strong active galactic nucleus signature is present. However, the luminosity of H₂O(2₁₁-2₀₂) and H₂O(2₂₀-2₁₁) appears to increase slightly faster than linear with L_IR. Although the slope turns out to be slightly steeper when z ∼ 2-4 ULIRGs are included, the correlation is still closely linear. We find that L_H2O/L_IRdecreases with increasing f₂₅/f₆₀, but see no dependence on f₆₀/f₁₀₀, possibly indicating that very warm dust contributes little to the excitation of the submillimeter H₂ O lines. The average spectral line energy distribution (SLED) of the entire sample is consistent with individual SLEDs and the IR pumping plus collisional excitation model, showing that the strongest lines are H₂O(2₀₂-1₁₁) and H₂O(3₂₁-3₁₂).