Observation of H2O in a strongly lensed Herschel-ATLAS source at z=2.3

Abstract

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 2(02)-1(11) 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 277-371 GHz receivers. The H2O line is detected at a redshift of z = 2.3049 +/- 0.0006, with a flux of 7.8 +/- 0.5 Jy km s(-1) and a FWHM of 250 +/- 60 km s(-1). The new flux is 2.4 times weaker than the previous tentative detection, although both remain marginally consistent within 1.6 sigma. The intrinsic line luminosity and rati! o of H2O(2(02)-1(11))/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 2(02)-1(11) 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.