Torbaniuk O. The mean transmission of the intergalactic medium using Lyα-forest in quasar spectra

The intergalactic medium is revealed by the numerous neutral Hydrogen HI absorption lines seen in the spectra of distant quasars, the so-called Lyα-forest. It traces the thermal and radiative history of the Universe as well as the evolution of underlying matter distribution over a wide range of scales and redshifts due to relation of the Lyα opacity of the intergalactic HI to its density and other physical parameters. As a measure of opacity the value F̄, named the mean transmission and defined as a ratio of observed (transmitted) and emitted fluxes, is used. One of the main problems is related to determination of emitted flux, i. e. the continuum level in quasar spectra. Determination of the continuum, or initial flux, in the quasar spectra prior to its absorption by the intergalactic HI is nontrivial problem, because it affects the precision of the mean transmission in the Lyα-forest, F̄(z). We compared the F̄(z) values obtained by various methods of the continuum determination. For this purpose we compiled all available F̄(z) data from the literature. It was found that the values of the F̄(z) obtained with the manually determined continuum are systematically higher than those obtained from extrapolated continuum. The difference varies from 5% at z = 2 up to 33% at z = 4.5, respectively. We developed a new method of determination of the continuum level, which based on the composite spectra of quasars with similar monochromatic luminosity at 1450 Å and similar spectral index within the wavelength range 1215–1450 Å. With this aim we created a new sample of 102 643 quasar spectra with redshifts 2.0 < z < 5.5 from Sloan Digital Sky Survey Data Release 10 (SDSS DR10). We determined the mean transmission of the intergalactic HI in the Lyα line as a function of the redshift z within the range 2.2 < z < 4.9. We studied properties of the quasar spectra using composite spectra stacked from subsamples of individual SDSS DR7 quasar spectra binned on spectral index, and logarithm of monochromatic luminosity at 1450 Å. We first found: a) a dependence of the emission line equivalent width on spectral index (correlation or anti correlation) for several lines, mostly for those lines for which the Baldwin effect is detected; b) there is no dependence between the virial mass of the central supermassive black hole of a quasar and its spectral index.