PRECIOUS METALS IN SDSS QUASAR SPECTRA. I. TRACKING THE EVOLUTION OF STRONG, 1.5 <z< 4.5 C IV ABSORBERS WITH THOUSANDS OF SYSTEMS

We have vastly increased the C iv statistics at intermediate redshift by surveying the thousands of quasars in the Sloan Digital Sky Survey (SDSS) Data-Release 7. We visually verified over 16,000 C iv systems with 1.46 < z < 4.55—a sample size that renders Poisson error negligible. Detailed Monte Carlo simulations show that we are approximately 50% complete down to rest equivalent widths W r ≈ 0.6 Å. We analyzed the sample as a whole and in 10 small redshift bins with approximately 1500 doublets each. The equivalent width frequency distributions f ( W r ) were well modeled by an exponential, with little evolution in shape. In contrast with previous studies that modeled the frequency distribution as a single power law, the fitted exponential gives a finite mass density for the C iv ions. The comoving line density evolved smoothly with redshift, increasing by a factor of 2.37 ± 0.09 from z = 4.55–1.96, then plateauing at for z = 1.96–1.46. Comparing our SDSS sample with z < 1 (ultraviolet) and z > 5 (infrared) surveys, we see an approximately 10-fold increase in over z ≈ 6 → 0, for W r ⩾ 0.6 Å. This suggests a monotonic and significant increase in the enrichment of gas outside galaxies over the 12 Gyr lifetime of the universe.