The Last Eight-Billion Years of Intergalactic C IV Evolution

We surveyed the Hubble Space Telescope UV spectra of 49 low-redshift quasars for z < 1 C IV candidates, relying solely on the characteristic wavelength separation of the doublet. After consideration of the defining traits of C IV doublets (e.g., consistent line profiles, other associated transitions, etc.), we defined a sample of 38 definite (group G = 1) and five likely (G = 2) doublets with rest equivalent widths W_r for both lines detected at ≥ 3σ_{W_{r}}. We conducted Monte Carlo completeness tests to measure the unblocked redshift (Δz) and co-moving path length (ΔX) over which we were sensitive to C IV doublets of a range of equivalent widths and column densities. The absorber line density of (G = 1+2) doublets is {d}{N}_{{C IV}}/{d}X= 4.1^{+0.7}_{-0.6} for log N(C⁺³) >= 13.2, and {d}{N}_{{C IV}}/{d}X has not evolved significantly since z = 5. The best-fit power law to the G = 1 frequency distribution of column densities f(N({C}^{+3})) ≡ k(N({C}^{+3})/N_{0})^{α_{N}} has coefficient k = 0.67^+0.18 _-0.16 × 10^-14 cm² and exponent α _N = -1.50^+0.17 _-0.19, where N ₀ = 10¹⁴ cm^-2. Using the power-law model of f(N(C⁺³)), we measured the C⁺³ mass density relative to the critical density: Ω _{{C}^{+3}}= (6.20^{+1.82}_{-1.52}) × 10^{-8} for 13 <= log N(C⁺³) <= 15. This value is a 2.8 ± 0.7 increase in Ω _{{C}^{+3}} compared to the error-weighted mean from several 1 < z < 5 surveys for C IV absorbers. A simple linear regression to Ω_{{C}^{+3}} over the age of the universe indicates that Ω_{{C}^{+3}} has slowly but steadily increased from z = 5 → 0, with {d}Ω_{{C}^{+3}}/ {d}t_age = (0.42± 0.2)× 10^{-8} Gyr^{-1}.