Evolution of CIV Absorbers I. The Cosmic Incidence

Abstract

We present a large high-resolution study of the distribution and evolution of CIV absorbers, including the weakest population with equivalent widths $W_r<0.3$~Å. By searching 369 high-resolution, high signal-to-noise spectra of quasars at $1.1\le z_{em}\le 5.3$ from Keck/HIRES and VLT/UVES, we find $1268$ CIV absorbers with $W_r\ge 0.05$~Å (our $\sim 50\%$ completeness limit) at redshifts $1\le z\le 4.75$. A Schechter function describes the observed equivalent width distribution with a transition from power-law to exponential decline at $W_r\gtrsim 0.5$~Å. The power-law slope $\alpha$ rises by $\sim 7\%$ and transition equivalent width $W_{\star }$ falls by $\sim 20\%$ from $⟨z⟩=1.7$ to $⟨z⟩=3.6$. We find that the co-moving redshift path density, $dN/dX$, of $W_r\ge 0.05$~Å absorbers rises by $\sim 1.8$ times from $z\simeq 4.0$ to $z\simeq 1.3$, while the $W_r\ge 0.6$~Å $dN/dX$ rises by a factor of $\sim 8.5$. We quantify the observed evolution by a model in which $dN/dX$ decreases linearly with 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 $z=3-5$ over-produce the observed abundance of absorbers with $W_r<0.3$~Å, while yielding better agreement at higher $W_r$. Our empirical linear model successfully describes {CIV evolution in the simulations and the observed evolution of $W_r\ge 0.6$~Å CIV for the past $\sim 12$ Gyr. Combining our measurements with the literature gives us a picture of CIV-absorbing structures becoming more numerous and/or larger in physical size over the last $\approx 13$ Gyr of cosmic time ($z\sim 6$ to $z\sim 0$).

27 pages, 13 figures, 7 tables; submitted to ApJ on July 22, 2020; revised September 28, 2020; accepted October 2, 2020