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Description
We report rest-frame submillimeter H_2_O 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 H_2_O emission lines. These include five 3_21_-3_12_ ortho-H_2_O lines (Eup/k=305K) and nine J=2 para-H_2_O lines, either 2_02_-1_11_ (E_up/k=101K) or 2_11_-2_02_ (E_up/k = 137K). The apparent luminosities of the H_2_O emission lines are {mu}L_H2O_~6-21x10^8^L_{sun}_ (3<{mu}<15, where {mu} is the lens magnification factor), with velocity-integrated line fluxes ranging from 4-15Jy.km/s. We have also observed CO emission lines using EMIR on the IRAM 30m telescope in seven sources (most of those have not yet had their CO emission lines observed). The velocity widths for CO and H_2_O lines are found to be similar, generally within 1{sigma} errors in the same source. With almost comparable integrated flux densities to those of the high-J CO line (ratios range from 0.4 to 1.1), H_2_O is found to be among the strongest molecular emitters in high-redshift Hy/ULIRGs. We also confirm our previously found correlation between luminosity of H_2_O (LH_2_O) and infrared (LIR) that LH_2_O~LIR^(1.1-1.2)^, with our new detections. This correlation could be explained by a dominant role of far-infrared pumping in the H_2_O excitation. Modelling reveals that the far-infrared radiation fields have warm dust temperature T_warm_~45-75K, H_2_O column density per unit velocity interval N_H2O_/{DELTA}V>~0.3x10^15^km/s/cm^2^ and 100{mu}m continuum opacity {tau}_100_>1 (optically thick), indicating that H_2_O is likely to trace highly obscured warm dense gas. However, further observations of J>=4 H_2_O lines are needed to better constrain the continuum optical depth and other physical conditions of the molecular gas and dust. We have also detected H_2_O^+^ emission in three sources. A tight correlation between L_H_2_O and L_H_2_O^+^ has been found in galaxies from low to high redshift. The velocity-integrated flux density ratio between H_2_O^+^ and H_2_O suggests that cosmic rays generated by strong star formation are possibly driving the H_2_O^+^ formation.
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