2016A&A...595A..80Y

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 (E_up/k=305K) and nine J=2 para-H₂O lines, either 2₀₂-1₁₁(E_up/k=101K) or 2₁₁-2₀₂(E_up/k=137K). The apparent luminosities of the H₂O emission lines are µL(H₂O)∼6-21x10⁸L_☉ (3<µ<15, where µ 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 30 m telescope in seven sources (most of those have not yet had their CO emission lines observed). The velocity widths for CO and H₂O lines are found to be similar, generally within 1σ 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), H₂O is found to be among the strongest molecular emitters in high-redshift Hy/ULIRGs. We also confirm our previously found correlation between luminosity of H₂O (L(H₂O)) and infrared (L_IR) that L(H₂O)∼L_IR^1.1–1.2, with ournew detections. This correlation could be explained by a dominant role of far-infrared pumping in the H₂O excitation. Modelling reveals that the far-infrared radiation fields have warm dust temperature T_warm∼45-75K, H₂O column density per unit velocity interval N(H₂O)/ΔV≥0.3x10¹⁵cm^–2km^–1 s and 100µm continuum opacity τ₁₀₀>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 L(H₂O) and L(H₂O⁺) 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.