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 . By searching 369 high-resolution, high signal-to-noise ratio spectra of quasars at from Keck/HIRES and VLT/UVES, we find 1268 C iv absorbers with Å (our ∼50% completeness limit) at redshifts . A Schechter function describes the observed equivalent width distribution with a transition from power-law to exponential decline at . The power-law slope α rises by ∼7%, and the transition equivalent width W ⋆ falls by ∼20% from to 3.6. We find that the comoving redshift path density, , of Å absorbers rises by ∼1.8 times from z ≃ 4.0 to 1.3, while the Å rises by a factor of ∼8.5. We quantify the observed evolution by a model in which decreases linearly with increasing redshift. The model suggests that populations with larger 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 while yielding better agreement at higher . Our empirical linear model successfully describes C iv evolution in the simulations and the observed evolution of Å C iv for the past . 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 of cosmic time (z ∼ 6–0).