2011A&A...530L...3O

The Herschel survey, H-ATLAS, with its large areal coverage, has recently discovered a number of bright, strongly lensed high-z submillimeter galaxies. The strong magnification makes it possible to study molecular species other than CO, which are otherwise difficult to observe in high-z galaxies. Among the lensed galaxies already identified by H-ATLAS, the source J090302.9-014127B (SDP.17b) at z=2.305 is remarkable because of its excitation conditions and a tentative detection of the H₂O 2₀₂-1₁₁ emission line (Lupu et al., 2010, ApJ, submitted). We report observations of this line in SDP.17b using the IRAM interferometer equipped with its new 277-371GHz receivers. The H₂O line is detected at a redshift of z=2.3049±0.0006, with a flux of 7.8±0.5Jy.km/s and a FWHM of 250±60km/s. The new flux is 2.4 times weaker than the previous tentative detection, although both remain marginally consistent within 1.6σ. The intrinsic line luminosity and ratio of H₂O(2₀₂-1₁₁)/CO(8-7) are comparable with those of the nearby starburst/enshrouded-AGN Mrk 231, and the ratio I(H₂O)/L_FIR is even higher, suggesting that SDP.17b could also host a luminous AGN. The detection of a strong H₂O 2₀₂-1₁₁ line in SDP.17b implies an efficient excitation mechanism of the water levels that must occur in very dense and warm interstellar gas probably similar to Mrk 231.