LYMAN-ALPHA ABSORPTION IN THE SPECTRUM OF THE Z = 4.5 QSO BR 1033-0327

We present the results of echelle spectroscopy of the Lyalpha forest r egion in the spectrum of the bright (m(R) = 18.5) optically selected z = 4.5 QSO BR 1033 - 0327. These results, which extend Lyalpha forest absorption line studies up to a redshift of z = 4.5 at a resolution of 12 km s-1, represent the first high-resolution study of Lyalpha fores t systems at such high redshifts. We compare the properties of the Lya lpha forest systems in the redshift range 3.7 < z < 4.3 with those inf erred from similar observations at 1.86 < z < 3.27 and find some clear indications of redshift dependence in these. First, the ionizing flux from the quasar appears to affect the Lyalpha line density further fr om the QSO than at lower redshifts, implying that the Lyman limit back ground flux at redshift z approximately 4.2 is J(v) approximately 1-3 x 10(-22) ergs cm-2 s-1 Hz-1 sr-1. This is about a factor of 10 lower than estimates at z approximately 2.5. Second, the Doppler parameter d istribution for systems where the effect of the QSO ionizing flux is s mall (3.7 < z < 4.3) has an excess at values approximately 20 km s-1, compared with lower redshifts; we suggest this may be due to the lower ionizing flux at z approximately 4.2 which would reduce the heating o f the gas. Finally, while there is no evidence that the shape of the H i column density distribution changes significantly, the redshift dep endence of the total number of systems is stronger than that found at lower redshifts, with the number of systems per unit redshift approxim ately (1 + z)4.6. Also, over the entire redshift range there is no sig nificant correlation between Doppler parameter and H I column density and no signal in the two-point correlation function down to velocity s eparations of DELTAv approximately 100 km s-1. There is no evidence fo r continuously distributed H I absorption (the Gunn-Peterson effect) a t these redshifts.