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 { \it 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 $\sim 1 \times 10^5$ $L_{ \odot}$ to $\sim 5 \times 10^7 L_{ \odot}$ while the total IR luminosities ($L_\mathrm{ IR}$) have a similar spread ($\sim 1-300 \times 10^{ 10} L_{ \odot}$). In addition, emission lines of H$_2$O$^+$ and H$_2^{ 18}$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 $L_\mathrm{ IR}$, no matter whether strong active galactic nucleus signature is present or not. 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 $L_\mathrm{ IR}$. Although the slope turns out to be slightly steeper when $z\sim 2-4$ ULIRGs are included, the correlation is still closely linear. We find that $L_\mathrm{ H_2O}/L_\mathrm{ IR}$ 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($3_{ 21}-3_{ 12}$).
Categories: astro-ph.GA astro-ph.CO