First detection of the 448 GHz ortho-H<sub>2</sub>O line at high redshift: probing the structure of a starburst nucleus at <i>z</i> = 3.63
C. Yang, E. González-Alfonso, A. Omont, M. Pereira-Santaella, J. Fischer, A. Beelen, R. Gavazzi
Abstract
Submillimeter rotational lines of H 2 O 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-H 2 O(4 23 − 3 30 line at 448 GHz, 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 448 GHz H 2 O(4 23 –3 30 ) line using ALMA in a strongly lensed submillimeter galaxy (SMG) at z = 3.63. After correcting for magnification, the luminosity of the 448 GHz H 2 O line is ∼10 6 L ⊙ . In combination with three other previously detected H 2 O 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 at 100 μ m τ 100 ∼ 0.3) disk component with a dust temperature T dust ≈ 50 K that emits a total infrared power of 5 × 10 12 L ⊙ with a surface density Σ IR = 4 × 10 11 L ⊙ kpc −2 , and a very compact (0.1 kpc) heavily dust-obscured ( τ 100 ≳ 1) nuclear core with very warm dust (100 K) and Σ IR = 8 × 10 12 L ⊙ kpc −2 . The H 2 O abundance in the core component, X H 2 O ∼ (0.3–5) × 10 −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 H 2 O are present in regions that are characterized by highly differing conditions and scales, extending from the nucleus to more extended regions of SMGs.