Lyman-alpha depression of the continuum from high-redshift quasars - A new technique applied in search of the Gunn-Peterson effect

We reexamine old and new data including the high-redshift (z > 4) quasar spectra of Schneider, Schmidt, & Gunn in search of opacity due to smoothly distributed hydrogen absorbing at the local wavelength of Lyα (the "Gunn-Peterson" effect). Following a detailed analysis of moderate redshift, high-resolution observations of the Lyα cloud population, we apply two methods to the data: (1) computing and subtracting off the mean absorption due to distinct clouds, and (2) a new method of examining the distribution of intensities in comparison with a Monte Carlo simulation. We find that at redshift z~3 the data imply approximately 30% more opacity than in measurable lines, which could be due to lines of very small column density or to true continuous absorption. At high redshift (s > 4), we find that the total absorption is larger by 20% +/- 30% than a simple extrapolation from lower redshift data. The most plausible interpretation of this is to set a limit of ~15% reduction in the ionizing background from z = 3 to z = 4.5. Models such as Hot Dark Matter, in which most matter is very smoothly distributed at redshifts z > 4, are marginally excluded by these results at the 2σ level.