Herschel strongly lensed submillimeter galaxies. H2O, a new diagnosis of their dense cores.

Herschel extragalactic wide surveys which have observed about 1000 deg^2, have discovered hundreds of high-redshift strongly lensed submillimeter galaxies. Gravitational magnification there allows a gain by an order of magnitude in sensitivity in studying submillimeter galaxies at high redshift (z ~ 1-5). I will first summarize the main results of these submm surveys, and our strategy for identifying, characterizing and following up these sources. Then I will focus on our H2O line studies in these lenses. After evoking Herschel results on H2O emission in local Ultra-Luminous Infrared Galaxies (ULIRGs, SFR > 100 Mo/yr), I will present our results on similar objects at high-z. Observing a sample of lenses from the H-ATLAS survey, we have shown that multi-line H2O is detectable in practically all Herschel lenses with the current sensitivity of IRAM-PdBI. We show that the H2O line luminosity increases more rapidly than L_IR. It thus reaches pretty high values in HLIRGs (SFR > 1000 Mo/yr), which have no local equivalents but are abundant at z ~ 2 - 4. H2O lines provide thus an important diagnosis in extreme starbust and possibly AGN conditions of warm, dense cores of these galaxies. However, modelling the infrared dominated excitation of H2O lines remains difficult. I will finally evoke various possible extensions of such studies, especially with ALMA.