Water Vapor in nearby Infrared Galaxies as Probed by Herschel

Abstract

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 H_(2)O emission line detected. The H_(2)O line luminosities range from ~1 × 10^5 L_☉ to ~5 × 10^7 L_☉ while the total IR luminosities (L_IR) have a similar spread (~1-300 × 10^10 L_☉). In addition, emission lines of H_(2)O^+ and H^(18)_(2)O are also detected. H_(2)O is found, for most galaxies, to be the strongest molecular emitter after CO in FTS spectra. The luminosity of the five most important H_(2)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 H_(2)O(2_11-2_02) and H_(2)O(2_20-2_11) 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_H_(2)O/L_IR decreases with increasing f_25/f_60, but see no dependence on f_60/f_100, possibly indicating that very warm dust contributes little to the excitation of the submillimeter H_(2)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 H_(2)O(2_02-1_11) and H_(2)O(3_21-3_12).

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