The Properties of Intergalactic C IV and Si IV Absorption, I: Optimal Analysis of an Extremely High S/N Quasar Sample

We have analyzed the properties of metals in the high redshift intergalactic medium using a novel objective pixel optical depth technique on a sample of extremely high signal-to-noise Keck HIRES and ESI spectra of 26 quasars between redshifts 2.1 and 6.4. The technique relies on using the doublet nature of the common ions CIV and SiIV that are the principal metal tracers in the intergalactic medium outside of the Ly alpha forest. Optical depths are statistically corrected for contamination by other lines, telluric absorption, bad pixels, continuum fitting, etc. and for incompleteness, and we achieve in this way an increased sensitivity of approximately 0.5 dex over previous analyses. Unlike existing POD techniques, we do not compare the ion optical depths with HI optical depths; we therefore avoid problems arising from different velocity widths in the ion and HI. We have shown how the conventional analysis can be reproduced using a percolation method to generate pseudo-clouds from ion optical depths. We find that for the higher resolution HIRES data there is a tight relation, tau ~ N^{0.7}, between the peak optical depth and the column density. From the optical depth vectors themselves we show that there 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. Two subsequent papers will investigate what fraction of the absorbers lie in galatic wind outflows (Paper II) and what metallicity is associated with regions of tau(Ly alpha) < 1 (Paper III).