LYMAN ALPHA EVOLUTION AT HIGH-RESOLUTION - EVIDENCE FOR A SINGLE POPULATION

Recent high-resolution spectroscopic data on the Lyman alpha absorption lines allow a statistical investigation of the cosmological evolution of Ly alpha-clouds from z = 1.8 to 3.8. The maximum likelihood analysis shows that the number density evolution is differential, being progressively slower for stronger lines. There are discrepancies in the derived evolution for lines with equivalent widths W > 0.3 angstrom from line samples obtained at high and intermediate resolutions. A possible explanation is given in terms of line blending. A differential evolution of this type seems to imply a unified evolutionary scheme which incorporates Ly-alpha and metal-line systems in a single population.