H2O emission in high-zultra-luminous infrared galaxies

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 14 Jy km s-1. The corresponding apparent luminosities are μLH2O ~ 3−12 × 108 L⊙, where μ is the lensing magnification factor (3  100 mJy) discovered in the Herschel surveys. Correcting the luminosities for amplification, using existing lensing models, LH2O is found to have a strong dependence on the infrared luminosity, varying as ~LIR1.2. 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 LIR ≳ 1013 L⊙ tend to be very strong emitters in water vapor, that have no equivalent in the local universe.