Cold Gas Kinematics in an L ∗ Spiral Galaxy at z = 0 . 437 : The Nature of Damped Lyman - α Absorbers

Westerbork Radio Synthesis Telescope observations of the redshifted 21cm line absorber against the z em = 0.871 double lobed quasar 3C196 show that the intervening absorber is an L ≈ L * spiral galaxy (3C196-G1) and that the absorbing layer of cold gas extends to radii of at least 30 h −1 50 kpc. The new data solve several long standing puzzles about this system by (1) discovering a second 21cm absorption feature, corresponding to absorption against the NE lobe of the background radio source and (2) spatially " resolving " the two absorption features to isolate the absorption along the two lines of sight to the opposing radio lobes. These findings resolve the disagreement in redshift between the UV metal and 21cm 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, rotating gas disk match the observed 21cm profile and are also compatible with both the redshift and velocity spread of the absorption measured in UV resonance lines along a third, independent line of sight to the quasar nucleus and with the lack of 21cm absorption in as earlier VLBI experiment that was sensitive to opacity against the hot spot in the northern lobe. The inferred rotation speed and luminosity for the galaxy are compatible with the z ≈ 0 Tully-Fisher Relation. This system illustrates well how 21cm absorption against extended background radio sources is a powerful tool in determining the nature of the damped Lyman-α class of QSO absorption line system.