Cold gas, the HI 21cm line and evolving galactic potentials
  Neutral hydrogen traces gravitational potentials. In the nearby universe, 21cm emission-line surveys show that the bulk of the HI resides in well-formed, optically-luminous galaxies. At high redshift, 21cm line absorption against background radio quasars occurs in gas-rich systems identified with the highest HI column densities -- the ``damped Lyman alpha'' quasar absorption-line systems. High spatial-resolution observations of the redshifted 21cm line absorbers measure sizes and kinematics of the neutral absorbers. 
F.H. Briggs