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Submillimeter rotational lines of H2O are a powerful probe in warm gasregions of the ISM, tracing scales and structures ranging from kpc disks to themost compact and dust-obscured regions of galactic nuclei. Theortho-H2O(423-330) line at 448 GHz, which was recently detected in a localluminous infrared galaxy (Pereira-Santaella et al. 2017), offers a uniqueconstraint on the excitation conditions and ISM properties in deeply buriedgalaxy nuclei since the line requires high far-IR optical depths to be excited.In this letter, we report the first high-redshift detection of the 448 GHzH2O(423-330) line using ALMA, in a strongly lensed submillimeter galaxy (SMG)at z=3.63. After correcting for magnification, the luminosity of the 448 GHzH2O line is ~10^6 L_sun. In combination with three other previously detectedH2O lines, we build a model that "resolves" the dusty ISM structure of the SMG,and find that it is composed of a ~1 kpc optically thin (optical depth at100{\mu}m {\tau}_{100}~0.3) disk component with dust temperature T_{dust}\approx 50 K emitting a total infrared power of 5e12 L_sun with surface density\Sigma_{IR}=4e11 L_sun kpc^{-2}, and a very compact (0.1 kpc) heavilydust-obscured ({\tau}_{100} \gtrsim 1) nuclear core with very warm dust (100 K)and \Sigma_{IR}=8e12 L_sun kpc^{-2}. The H2O abundance in the core component,X_{H2O}~(0.3-5)e{-5}, is at least one order of magnitude higher than in thedisk component. The optically thick core has the characteristic properties ofan Eddington-limited starburst, providing evidence that radiation pressure ondust is capable of supporting the ISM in buried nuclei at high redshifts. Themulti-component ISM structure revealed by our models illustrates that dust andmolecules such as H2O are present in regions characterized by highly differingconditions and scales, extending from the nucleus to more extended regions ofSMGs.

First detection of the 448 GHz ortho-H2O line at high redshift: probing the structure of a starburst nucleus at z = 3.63