Submillimeter H2O and H2O+ emission in lensed ultra- and hyper-luminous infrared galaxies at z ~ 2–4

We report rest-frame submillimeter H2O emission line observations of 11 ultra- or hyper-luminous infrared galaxies (ULIRGs or HyLIRGs) at z ~ 2–4 selected among the brightest lensed galaxies discovered in the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS). Using the IRAM NOrthern Extended Millimeter Array (NOEMA), we have detected 14 new H2O emission lines. These include five 321–312ortho-H2O lines (Eup/k = 305 K) and nine J = 2 para-H2O lines, either 202–111(Eup/k = 101 K) or 211–202(Eup/k = 137 K). The apparent luminosities of the H2O emission lines are μLH2O ~ 6–21 × 108 L⊙  (3 1 (optically thick), indicating that H2O is likely to trace highly obscured warm dense gas. However, further observations of J ≥ 4 H2O lines are needed to better constrain the continuum optical depth and other physical conditions of the molecular gas and dust. We have also detected H2O+ emission in three sources. A tight correlation between LH2O and LH2O+ has been found in galaxies from low to high redshift. The velocity-integrated flux density ratio between H2O+ and H2O suggests that cosmic rays generated by strong star formation are possibly driving the H2O+ formation.