Cold gas kinematics in an L-* spiral Galaxy at z=0.437: The nature of damped Lyman-alpha absorbers

Westerbork Radio Synthesis Telescope observations of the redshifted 21 cm l ine absorber against the z(em) = 0.871 double lobed quasar 3C 196 show that the intervening absorber is an L approximate toL(*) spiral galaxy (3C 196- G1) and that the absorbing layer of cold gas extends to radii of at least 3 0 h(50)(-1) The new data solve several long standing puzzles about this sys tem by (1) discovering a second 21 cm absorption feature, corresponding to absorption against the NE lobe of the background radio source and (2) spati ally "resolving" the two absorption features to isolate the absorption alon g the two lines of sight to the opposing radio lobes. These findings resolv e the disagreement in redshift between the UV metal and 21 cm lines, and as well as demonstrating that the neutral layer does absorb both lobes of the background radio source. Simple kinematic models with an inclined, rotatin g gas disk match the observed 21 cm profile and are also compatible with bo th the redshift and velocity spread of the absorption measured in UV resona nce lines along a third, independent line of sight to the quasar nucleus an d with the lack of 21 cm absorption in as earlier VLBI experiment that was sensitive to opacity against the hot spot in the northern lobe. The inferre d rotation speed and luminosity for the galaxy are compatible with the z ap proximate to 0 Tully-Fisher Relation. This system illustrates well how 21 c m absorption against extended background radio sources is a powerful tool i n determining the nature of the damped Lyman-alpha class of QSO absorption line system.