Evolution of CIV 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 < 0.3 angstrom. 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 withW(r) > 0.05 angstrom (our similar to 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 greater than or similar to 0.5 angstrom. The power-law slope alpha rises by similar to 7%, and the transition equivalent width Wa falls by similar to 20% from azn = 1.7 to 3.6. We find that the comoving redshift path density, dN dX, of W-r >= 0.05 angstrom absorbers rises by similar to 1.8 times from z similar or equal to 4.0 to 1.3, while the W-r >= 0.6 angstrom dN dX rises by a factor of similar to 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 Wr 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 <= 0.3 angstrom 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 >= 0.6 angstrom C IV for the past similar to 12 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 approximate to 13 Gyr of cosmic time (z similar to 6-0).