We present a kinematic model of the neutral-hydrogen in the spiral galaxy of the lens system PKS1830 - 211 , based on a Multi Element Radio-Linked Interferometer ( MERLIN ) 1.4–GHz radio map and the integrated and redshifted 21–cm hydrogen absorption-line profile as measured with the Westerbork Synthesis Radio Telescope ( WSRT ) .
Degeneracies in the models do not allow a unique determination of the kinematic center and forthcoming deeper Hubble Space Telescope observations with the Advanced Camera for Surveys ( ACS ) are required to break this degeneracy .
Even so , we measure the inclination of the hydrogen disk : i = ( 17 - 32 ) ^ { \circ } , indicating a close-to face-on spiral galaxy .
The optical depth increases with radius over the extent of the Einstein ring , suggesting HI depletion towards the lens center .
The latter could be due to star formation or conversion of HI in to molecular hydrogen because of a higher metalicity/dust content in the galaxy center .
The neutral hydrogen optical depth gives N _ { HI } = 2 \times 10 ^ { 21 } cm ^ { -2 } at r = 5.0 h ^ { -1 } _ { 70 } kpc in the disk ( T _ { s } = 100 K ) , comparable to local spiral galaxies .
Our study shows that planned new radio telescopes ( i.e .
ALMA , LOFAR and SKA ) – which will discover large numbers of similar lens systems – are powerful new tools to probe the internal structure , kinematics and evolution of spiral galaxies in detail to z \gtrsim 1 , as well as their neutral-hydrogen ( and molecular ) content , thereby complementing studies of HI emission from spirals at z \lesssim 1 and damped Ly– \alpha systems to z \gg 1 .