H2O emission in high-z ultra-luminous infrared galaxies

TL;DR

This study reports the detection of water emission lines in six high-redshift ultra-luminous infrared galaxies, revealing strong H2O emission correlated with IR luminosity, and demonstrating the feasibility of detecting such lines with current instruments.

Contribution

First detection of H2O emission in multiple high-z ULIRGs, establishing its strength and relation to IR luminosity, and highlighting the potential of Herschel and IRAM PdBI for studying molecular lines in distant galaxies.

Findings

H2O lines are among the strongest molecular lines in high-z ULIRGs.

H2O luminosity correlates with IR luminosity as ~L_IR^{1.2}.

H2O can be detected in high-z lensed galaxies with current sensitivity.

Abstract

Using IRAM PdBI we report the detection of H2O in six new lensed ultra-luminous starburst galaxies at high redshift, discovered in the Herschel H-ATLAS survey. 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. The corresponding apparent luminosities are mu x L_H2O ~ 3-12 x 10^8 Lo, where mu is the lensing magnification factor (3 < mu < 12). These results confirm that H2O lines are among the strongest molecular lines in such galaxies, with intensities almost comparable to those of the high-J CO lines, and same profiles and line widths (200-900 km/s) as the latter. With the current sensitivity of PdBI, H2O can therefore easily be detected in high-z lensed galaxies (with F(500um) > 100 mJy) discovered in the Herschel surveys. Correcting the luminosities for lensing amplification, L_H2O is found to have a strong dependence on the IR luminosity, varying as ~L_IR^{1.2}. This relation which needs to be confirmed with better statistics, may indicate a role of radiative (IR) excitation of the H2O lines, and implies that high-z galaxies with L_IR >~ 10^13 Lo tend to be very strong emitters in H2O, that have no equivalent in the local universe.