The multiple coherence scales of C IV at cosmic noon

TL;DR

This work probes the spatial and kinematic structure of the CGM at cosmic noon by analyzing CIV absorption in a large collection of closely separated quasar pairs. Through Voigt-profile decomposition and robust two-point statistics, the authors detect 141 CIV systems and measure the correlation function $\xi(\Delta v,\Delta r)$ and its projected form $\Xi(\Delta r)$ across sub-kpc to Mpc scales, uncovering a sharp small-scale rise and a flattening at intermediate separations. Fitting the projected function with broken power-law models reveals two coherence lengths: a large-scale CIV-enriched region scale $r_1 \approx 6\times10^{2}$ kpc and a small-scale CIV-cloud scale $r_2 \approx 4.7$ kpc, suggesting distinct CGM environments within which CIV resides. The results indicate that CIV traces both the extended clustering of galaxies at $z\sim2$ and the internal, few-kiloparsec coherence of metal-enriched clouds, providing new constraints on metal transport, CGM structure, and the baryon cycle during the peak era of galaxy formation.

Abstract

The spatial and kinematic structure of the circumgalactic medium (CGM) remains poorly constrained observationally. In this article we compute the clustering of CIV absorption systems at cosmic noon using quasar pairs. We analyze VLT/UVES and Keck/HIRES high-resolution spectra (R = 45000) of a sample of 8 projected and 4 lensed quasar pairs that probe transverse separations, $Δr$, from sub-kpc to a few Mpc, over the redshift range 1.6 < z < 3.3. We detect and fit Voigt profiles to a total of 141 CIV systems, corresponding to 620 velocity components across all quasar lines-of-sight. We compute the two-point correlation function of CIV, $ξ(Δv, Δr)$, where $Δv$ is the velocity difference between components across all available scales. We find a strong dependence of $ξ(Δr)$ with $Δr$ at all velocities. $ξ(Δr)$ reaches a sharp peak at the smallest scales analyzed here, $Δr\approx 0.1$ kpc, decreases steadily up to $Δr\approx 5$ kpc and remains flat up to $Δr\approx 500$ kpc, where it begins to decrease again. By fitting power-laws to the projected transverse correlation function $Ξ(Δr)$, we infer two coherence lengths: $r_1 = 654^{+100}_{-87}$ kpc, which we interpret as a representative size for the CIV enriched regions at $z\approx 2$, and $r_2 = 4.70^{+1.60}_{-1.19}$ kpc for the individual CIV-bearing "clouds". Projecting instead in $Δr$, we find consistent amplitudes of $ξ(Δv)$ with previous work using quasars and extended background sources. Our results suggest that CIV may be a good tracer of not only the small, internal structure of the circumgalactic medium, but also of the way in which galaxies cluster at cosmic noon.

Figures (9)

• Figure 1: Redshift path ($\Delta z$) and completeness for line-of-sight A (B) in black (green) as a function of column density threshold ($N_{min}$).
• Figure 2: Examples of Civ absorption systems fitted with Voigt profiles. Left panel: absorption system at $z_{abs} = 2.2008$ towards the lensed quasar HE 1104-1805. Right panel: absorption system at $z_{abs} = 2.43$ towards J234628+124858 and J234625+124743. Top (bottom) panels correspond to line-of-sight A (B). The black histogram corresponds to the normalized flux of the quasars, and the red lines correspond to the Voigt profile fit of the system. The velocity zero-point was chosen by eye around the 1548 Å Civ line.
• Figure 3: Civ column density distribution for lines-of-sight A in black and lines-of-sight B in green. The pink solid line corresponds to the power law fit by Kim13 to data of 18 VLT/UVES single quasar sightlines, while the shaded area is the 1$\sigma$ uncertainty in the fit.
• Figure 4: Distribution of Civ velocity components. The blue histogram corresponds to velocity components found on lensed quasars pairs, while the red histogram corresponds to velocity components found on projected quasar pairs.
• Figure 5: Two-point correlation function ($\xi(\Delta r)$) of Civ as a function of line-of-sight separation for 3 different bins of velocity. Red and blue points are slightly shifted for clarity from their identical position to the blue points. Open circle data points correspond to the measurements obtained by Martin10.