The Properties of Intergalactic Civand SiivAbsorption. I. Optimal Analysis of an Extremely High Signal-to-Noise Quasar Sample

We have analyzed the properties of metals in the high redshift intergalacticmedium using a novel objective pixel optical depth technique on a sample ofextremely high signal-to-noise Keck HIRES and ESI spectra of 26 quasars betweenredshifts 2.1 and 6.4. The technique relies on using the doublet nature of thecommon ions CIV and SiIV that are the principal metal tracers in theintergalactic medium outside of the Ly alpha forest. Optical depths arestatistically corrected for contamination by other lines, telluric absorption,bad pixels, continuum fitting, etc. and for incompleteness, and we achieve inthis way an increased sensitivity of approximately 0.5 dex over previousanalyses. Unlike existing POD techniques, we do not compare the ion opticaldepths with HI optical depths; we therefore avoid problems arising fromdifferent velocity widths in the ion and HI. We have shown how the conventionalanalysis can be reproduced using a percolation method to generate pseudo-cloudsfrom ion optical depths. We find that for the higher resolution HIRES datathere is a tight relation, tau ~ N^{0.7}, between the peak optical depth andthe column density. From the optical depth vectors themselves we show thatthere is little evolution in the total amount of CIV from z = 2 to z = 5,though there is a turndown of at least a factor of two in Omega(CIV) above z =5. We do, however, see substantial evolution in the ratio, SiIV/CIV. Twosubsequent papers will investigate what fraction of the absorbers lie ingalatic wind outflows (Paper II) and what metallicity is associated withregions of tau(Ly alpha) < 1 (Paper III).