THE LAST EIGHT-BILLION YEARS OF INTERGALACTIC C IV EVOLUTION

Abstract

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 Wr for both lines detected at $\ge 3\sigma _{W_{r}}$. 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 ${d}\mathcal {N}_{\mathrm{C\,IV}}/{d}X= 4.1^{+0.7}_{-0.6}$ for log N(C+3) ≥ 13.2, and ${d}\mathcal {N}_{\mathrm{C\,IV}}/{d}X$ has not evolved significantly since z = 5. The best-fit power law to the G = 1 frequency distribution of column densities $f(N(\mathrm{C}^{+3})) \equiv k(N(\mathrm{C}^{+3})/N_{0})^{\alpha _{N}}$ 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: $\Omega _{\mathrm{C}^{+3}}= (6.20^{+1.82}_{-1.52}) \times 10^{-8}$ for 13 ≤ log N(C+3) ≤ 15. This value is a 2.8 ± 0.7 increase in $\Omega _{\mathrm{C}^{+3}}$ compared to the error-weighted mean from several 1 < z < 5 surveys for C IV absorbers. A simple linear regression to $\Omega _{\mathrm{C}^{+3}}$ over the age of the universe indicates that $\Omega _{\mathrm{C}^{+3}}$ has slowly but steadily increased from z = 5 → 0, with ${d}\Omega _{\mathrm{C}^{+3}}/ {d}t_{\rm age} = (0.42\pm 0.2)\times 10^{-8}\,{\rm Gyr}^{-1}$.