Yang, C., González Alonso, E., Omont, A., Pereira Santaella, M., Fischer, J,. Beeleng, A., Gavazzi, R. (2020). First detection of the 448 GHz ortho-H2O line at high redshift: probing the structure of a starburst nucleus at z=3.63. Astronomy and Astrophysics 634 DOI: 10.1051/0004-6361/201937319
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(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 H2O(4(23)-3(30)) line using ALMA in a strongly lensed submillimeter galaxy (SMG) at z& x2004;=& x2004;3.63. After correcting for magnification, the luminosity of the 448 GHz H2O line is similar to 10(6)& x2006;L-circle dot. 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 similar to 1 kpc optically thin (optical depth at 100 mu m tau(100)& x2004;similar to& x2004;0.3) disk component with a dust temperature T-dust& x2004;approximate to& x2004;50 K that emits a total infrared power of 5 x 10(12)& x2006;L-circle dot with a surface density sigma(IR)& x2004;=& x2004;4 x 10(11)& x2006;L-circle dot kpc(-2), and a very compact (0.1 kpc) heavily dust-obscured (tau(100)& x2004;greater than or similar to& x2004;1) nuclear core with very warm dust (100 K) and sigma(IR)& x2004;=& x2004;8 x 10(12)& x2006;L-circle dot kpc(-2). The H2O abundance in the core component, XH2O& x2004;similar to& x2004;(0.3-5) x 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 H2O are present in regions that are characterized by highly differing conditions and scales, extending from the nucleus to more extended regions of SMGs.