2013A&A...551A.115O

Using the IRAM Plateau de Bure interferometer (PdBI), we report the detection of water vapor in six new lensed ultra-luminous starburst galaxies at high redshift, discovered in the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). The sources are detected either in the 2₀₂-1₁₁ or 2₁₁-2₀₂ H₂O emission lines with integrated line fluxes ranging from 1.8 to 14Jy.km/s. The corresponding apparent luminosities are µL_H2_O∼3-12x10⁸L_☉, where µ is the lensing magnification factor (3<µ<12). These results confirm that H₂O lines are among the strongest molecular lines in high-z ultra-luminous starburst galaxies, with intensities almost comparable to those of the high-J CO lines, and similar profiles and line widths (∼200-900km/s). With the current sensitivity of the PdBI, the water lines can therefore easily be detected in high-z lensed galaxies (with F(500µm)>100mJy) discovered in the Herschel surveys. Correcting the luminosities for amplification, using existing lensing models, L_H2_O is found to have a strong dependence on the infrared luminosity, varying as ∼L_IR^1.2. This relation, which needs to be confirmed with better statistics, may indicate a role of radiative (infrared) excitation of the H₂O lines, and implies that high-z galaxies with L_IR>10¹³L_☉ tend to be very strong emitters in water vapor, that have no equivalent in the local universe.