H_2O emission in high-z ultra-luminous infrared galaxies

Abstract

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 2_(02)−1_(11) or 2_(11)−2_(02) H_2O emission lines with integrated line fluxes ranging from 1.8 to 14 Jy km s^(-1). The corresponding apparent luminosities are μL_(H2O) ~ 3−12 × 10^8 L_⊙, where μ is the lensing magnification factor (3 < μ < 12). These results confirm that H_2O 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 (~200−900 km s^(-1)). With the current sensitivity of the PdBI, the water lines can therefore easily be detected in high-z lensed galaxies (with F(500 μm) > 100 mJy) discovered in the Herschel surveys. Correcting the luminosities for amplification, using existing lensing models, L_(H2O) 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 H_2O lines, and implies that high-z galaxies with L_(IR) ≳ 10^(13) L_⊙ tend to be very strong emitters in water vapor, that have no equivalent in the local universe.