Submillimeter H2O and H2O+ emission in lensed ultra- and hyper-luminous
  infrared galaxies at z ~ 2-4
  (abridged) We report rest-frame submillimeter H2O emission line observations
of 11 HyLIRGs/ULIRGs at z~2-4 selected among the brightest lensed galaxies
discovered in the Herschel-ATLAS. Using the IRAM NOEMA, we have detected 14 new
H2O emission lines. The apparent luminosities of the H2O emission lines are
$\mu L_{\rm{H_2O}} \sim 6-21 \times 10^8 L_\odot$, with velocity-integrated
line fluxes ranging from 4-15 Jy km s$^{-1}$. We have also observed CO emission
lines using EMIR on the IRAM 30m telescope in seven sources. The velocity
widths for CO and H2O lines are found to be similar. With almost comparable
integrated flux densities to those of the high-J CO line, H2O is found to be
among the strongest molecular emitters in high-z Hy/ULIRGs. We also confirm our
previously found correlation between luminosity of H2O ($L_{\rm{H_2O}}$) and
infrared ($L_{\rm{IR}}$) that $L_{\rm{H_2O}} \sim L_{\rm{IR}}^{1.1-1.2}$, with
our new detections. This correlation could be explained by a dominant role of
far-infrared (FIR) pumping in the H2O excitation. Modelling reveals the FIR
radiation fields have warm dust temperature $T_\rm{warm}$~45-75 K, H2O column
density per unit velocity interval $N_{\rm{H_2O}}/\Delta V \gtrsim 0.3 \times
10^{15}$ cm$^{-2}$ km$^{-1}$ s and 100 $\mu$m continuum opacity $\tau_{100} >
1$ (optically thick), indicating that H2O is likely to trace highly obscured
warm dense gas. However, further observations of $J\geq4$ 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 $L_{\rm{H_2O}}$ and $L_{\rm{H_2O^+}}$ 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.
