H2O emission in high-z ultra-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 μL_H2O ~ 3-12 × 10⁸ L_⊙, 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 (~200-900 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 H₂O lines, and implies that high-z galaxies with L_IR ≳ 10¹³ L_⊙ tend to be very strong emitters in water vapor, that have no equivalent in the local universe.

Herschel (Pilbratt et al. 2010) is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.