Assigned ID: 798 ----- Attempting Submission 1 (Fri Feb 24 22:14:22 GMT 2012) ----- We propose to study the evolution of intergalactic HI over the past 12 Gyrs, by combining our recent results from high quality ground-based observations covering the intergalactic gas at 1.5 < z < 4 with highest quality archival HST observations from STIS (1.0 < z < 1.3) and COS (0 < z < 0.48). The HST archive data set includes 28 COS spectra with S/N > 25 and 2 STIS spectra with S/N ~ 15. The evolution of the HI in the intergalactic medium is governed by cosmic expansion which cools the gas, the gravitational assembly of matter which heats the gas, and the mechanical and radiative feedback from galaxies and AGNs. Our primary scientific goals are to measure 1) the evolution of the HI absorption line number density from z = 0 to 4 in order to constrain the evolution of the UV ionizing radiation field; 2) the evolution of the HI line width to constrain the change of physical and kinematical conditions in the intergalactic gas as the Universe evolves. QUASAR ABSORPTION LINES AND IGM 12 Medium false 0.0 0.0 false false false false SUPPORTED