The Minimum Universal Metal Density between Redshifts of 1.5 and 5.5

It appears that the Lyman alpha forest is becoming thick at a redshift ofabout 5.5, cutting off the higher redshift intergalactic medium from view inneutral hydrogen. However, the effects of star formation at higher redshift arestill readable in the intergalactic metal lines. In this paper I useobservations of 32 quasars with emission redshifts in the range 2.31 to 5.86 tostudy the evolution of the intergalactic metal density from z = 1.5 to z = 5.5.The C IV column density distribution function is consistent with beinginvariant throughout this redshift range. From direct integration, I determineOmega_CIV to be in the range (2.5 - 7) \times 10^{-8} and Omega_SiIV in therange (0.9 - 3) \times 10^{-8} between z = 1.5 and z = 5. The metallicity at z= 5 exceeds 3.5 \times 10^{-4}, which in turn implies that this fraction of theuniversal massive star formation took place beyond this redshift. This issufficient to have ionized the intergalactic medium.Comment: This posting (v3) adds a factor of (1/c) in equation (1), and also the definition of Delta X, both omitted from v2. This is a typographical error and there is no impact on the published values of Omega_ion in the text and figures. v2: Revised to correct an error in equation (1) in the published version. This was a typographical error only and all values of Omega_ion in the text and figures of the published version were correctly computed with H_0 = 65 km/s/Mpc and Omega_m = 1. (5 pages including 4 figures.