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 } \text { - - } 3 _ { 30 } ) line at 448 GHz , which was recently detected in a local luminous infrared galaxy ( ) , offers a unique constraint on the excitation conditions and ISM properties in deeply buried galaxy nuclei since 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 } \text { - - } 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 \sim 10 ^ { 6 } L _ { \odot } .
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 \sim 1 kpc optically thin ( optical depth at 100 \mu m \tau _ { 100 } \sim 0.3 ) disk component with dust temperature T _ { dust } \approx 50 K emitting a total infrared power of 5 \times 10 ^ { 12 } L _ { \odot } with surface density \Sigma _ { \mathrm { IR } } = 4 \times 10 ^ { 11 } L _ { \odot } kpc ^ { -2 } , and a very compact ( 0.1 kpc ) heavily dust-obscured ( \tau _ { 100 } \gtrsim 1 ) nuclear core with very warm dust ( 100 K ) and \Sigma _ { \mathrm { IR } } = 8 \times 10 ^ { 12 } L _ { \odot } kpc ^ { -2 } .
The H 2 O abundance in the core component , X _ { \mathrm { H _ { 2 } O } } \sim ( 0.3 \text { - - } 5 ) \times 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 multi-component ISM structure revealed by our models illustrates that dust and molecules such as H 2 O are present in regions characterized by highly differing conditions and scales , extending from the nucleus to more extended regions of SMGs .