2020ApJ...904...44H

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< 0.3Å. By searching 369 high-resolution, high signal-to-noise ratio spectra of quasars at 1.1≤ z_em≤ 5.3 from Keck/HIRES and VLT/UVES, we find 1268 C IV absorbers with W_r≥ 0.05 Å (our ∼50% completeness limit) at redshifts 1.0≤ z≤ 4.75. A Schechter function describes the observed equivalent width distribution with a transition from power-law to exponential decline at W_r≳0.5Å. The power-law slope α rises by ∼7%, and the transition equivalent width W_* falls by ∼20% from <z>=1.7 to 3.6. We find that the comoving redshift path density, dN/dX, of W_r≥ 0.05 Å absorbers rises by ∼1.8 times from z ≃ 4.0 to 1.3, while the W_r≥ 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≤ 0.3Åwhile 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≥ 0.6 Å C IV for the past ∼12Gyr. 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 ~13Gyr of cosmic time (z ∼ 6-0).