HI kinematics in a massive spiral galaxy at z=0.89

Abstract

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) deg., 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= 2x10^21 cm^-2 at r=5.0 h_{70}^-1 kpc in the disk (T_s=100K), 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>=1, as well as their neutral-hydrogen (and molecular) content, thereby complementing studies of HI emission from spirals at z<=1 and damped Ly-alpha systems to z>>1.