2001ApJ...561L.153S

It appears that the Lyα forest is becoming thick at a redshift of about 5.5, cutting off the higher redshift intergalactic medium from view in neutral hydrogen. However, the effects of star formation at higher redshift are still readable in the intergalactic metal lines. In this Letter, I use observations of 32 quasars with emission redshifts in the range 2.31-5.86 to study 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 being invariant throughout this redshift range. From direct integration, I determine Ω_CIV to be in the range (2.5-7)x10^–8 and Ω_SiIV in the range (0.9-3)x10^–8 between z=1.5 and z=5. The metallicity at z=5 exceeds 3.5x10^–4, which in turn implies that this fraction of the universal massive star formation took place beyond this redshift. This is sufficient to have ionized the intergalactic medium.