2020A&A...634L...3Y

Submillimeter rotational lines of H₂O are a powerful probe in warm gas regions of the interstellar medium (ISM), tracing scales and structures ranging from kiloparsec disks to the most compact and dust-obscured regions of galactic nuclei. The ortho-H₂O(4₂₃-3₃₀ line at 448GHz, which has recently been detected in a local luminous infrared galaxy, offers a unique constraint on the excitation conditions and ISM properties in deeply buried galaxy nuclei because the line requires high far-infrared optical depths to be excited. In this letter, we report the first high-redshift detection of the 448GHz H₂O(4₂₃-3₃₀) line using ALMA in a strongly lensed submillimeter galaxy (SMG) at z=3.63. After correcting for magnification, the luminosity of the 448GHz H₂O line is ∼10⁶L_☉. In combination with three other previously detected H₂O lines, we build a model that resolves the dusty ISM structure of the SMG, and find that it is composed of a ∼1kpc optically thin (optical depth at 100µm τ₁₀₀∼0.3) disk component with a dust temperature T_dust~=50K that emits a total infrared power of 5x10¹²L_☉ with a surface density Σ_IR=4x10¹¹L_☉/kpc², and a very compact (0.1kpc) heavily dust-obscured (τ₁₀₀≥1) nuclear core with very warm dust (100K) and Σ_IR=8x10¹²L_☉/kpc². The H₂O abundance in the core component, X_H2O~(0.3-5)x10^–5, is at least one order of magnitude higher than in the disk component. The optically thick core has the characteristic properties of an Eddington-limited starburst, providing evidence that radiation pressure on dust is capable of supporting the ISM in buried nuclei at high redshifts. The multicomponent ISM structure revealed by our models illustrates that dust and molecules such as H₂O are present in regions that are characterized by highly differing conditions and scales, extending from the nucleus to more extended regions of SMGs.