Water Vapor in Nearby Infrared Galaxies as Probed by Herschel

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 H2O emission line detected. The H2O line luminosities range from ∼ 1× 105 L⊙ to ∼ 5× 10L⊙ while the total IR luminosities (LIR) have a similar spread (∼ 1 − 300× 10L⊙). In addition, emission lines of H2O and H18 2 O are also detected. H2O is found, for most galaxies, to be the strongest molecular emitter after CO in FTS spectra. The luminosity of the five most important H2O lines is near-linearly correlated with LIR, no matter whether strong active galactic nucleus signature is present or not. However, the luminosity of H2O(211 − 202) and H2O(220 − 211) appears to increase slightly faster than linear with LIR. 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 LH2O/LIR decreases 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 H2O 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).