Evolution of C IV Absorbers. I. The Cosmic Incidence

We present a large high-resolution study of the distribution and evolution of C IV absorbers, including the weakest population with equivalent widths ${W}_{r}\lt 0.3\,\mathring{\rm A} $ . By searching 369 high-resolution, high signal-to-noise ratio spectra of quasars at $1.1\leqslant {z}_{\mathrm{em}}\leqslant 5.3$ from Keck/HIRES and VLT/UVES, we find 1268 C IV absorbers with ${W}_{r}\geqslant 0.05$ Å (our ∼50% completeness limit) at redshifts $1.0\leqslant z\leqslant 4.75$ . A Schechter function describes the observed equivalent width distribution with a transition from power-law to exponential decline at ${W}_{r}\gtrsim 0.5\,\mathring{\rm A} $ . The power-law slope α rises by ∼7%, and the transition equivalent width W_⋆ falls by ∼20% from $\langle z\rangle =1.7$ to 3.6. We find that the comoving redshift path density, ${dN}/{dX}$ , of ${W}_{r}\geqslant 0.05$ Å absorbers rises by ∼1.8 times from z ≃ 4.0 to 1.3, while the ${W}_{r}\geqslant 0.6$ Å ${dN}/{dX}$ rises by a factor of ∼8.5. We quantify the observed evolution by a model in which ${dN}/{dX}$ decreases linearly with increasing redshift. The model suggests that populations with larger ${W}_{r}$ thresholds evolve faster with redshift and appear later in the universe. The cosmological TECHNICOLOR DAWN simulations at z = 3-5 overproduce the observed abundance of absorbers with ${W}_{r}\leqslant 0.3\,\mathring{\rm A} $ while yielding better agreement at higher ${W}_{r}$ . Our empirical linear model successfully describes C IV evolution in the simulations and the observed evolution of ${W}_{r}\geqslant 0.6$ Å C IV for the past $\sim 12\,\mathrm{Gyr}$ . Combining our measurements with the literature gives us a picture of C IV absorbing structures becoming more numerous and/or larger in physical size over the last $\approx 13\,\mathrm{Gyr}$ of cosmic time (z ∼ 6-0).