A downturn in intergalactic C iv as redshift 6 is approached

Abstract

We present the results of the largest survey to date for intergalactic metals at redshifts z > 5, using near‐IR spectra of nine quasi‐stellar objects with emission redshifts zem > 5.7. We detect three strong C iv doublets at zabs= 5.7–5.8, two low‐ionization systems at zabs > 5 and numerous Mg ii absorbers at zabs= 2.5–2.8. We find, for the first time, a change in the comoving mass density of C iv ions as we look back to redshifts z > 5. At a mean 〈z〉= 5.76, we deduce ΩC iv= (4.4 ± 2.6) × 10−9 which implies a drop by a factor of ∼3.5 compared to the value at z < 4.7, after accounting for the differing sensitivities of different surveys. The observed number of C iv doublets is also lower by a similar factor, compared to expectations for a non‐evolving column density distribution of absorbers. These results point to a rapid build‐up of intergalactic C iv over a period of only ∼300 Myr; such a build‐up could reflect the accumulation of metals associated with the rising levels of star formation activity from z∼ 9 indicated by galaxy counts, and/or an increasing degree of ionization of the intergalactic medium (IGM), following the overlap of ionization fronts from star‐forming regions. If the value of ΩC iv we derive is typical of the IGM at large, it would imply a metallicity ZIGM≳ 10−4 Z⊙. The early‐type stars responsible for synthesizing these metals would have emitted only about one Lyman continuum photon per baryon prior to z= 5.8; such a background is insufficient to keep the IGM ionized and we speculate on possible factors which could make up the required shortfall.

CONSTRAINTS ON THE UNIVERSAL C IV MASS DENSITY ATz∼ 6 FROM EARLY INFRARED SPECTRA OBTAINED WITH THE MAGELLAN FIRE SPECTROGRAPH

“…Inspection of the Voigt profile Doppler parameters in Table 2 shows a high average b, similar to what was described in Ryan-Weber et al (2009). This width is more likely to represent the full velocity spread of a suite of narrow components, rather than a large, single component.…”

“…Broadly speaking, we recover most lines (>80%) above N C iv = 10 14 and relatively few (<10%) below N C iv = 10 13.2 , with the 50% level occurring near 10 13.7 . Thus our survey sits between the parameter space explored by Becker et al (2009), who are complete to lower N C iv over a significantly smaller path length, and Ryan-Weber et al (2009) who have a comparable path length but slightly lower completeness.…”

“…These pilot studies yielded similar values of Ω C iv as at lower z, but with few actual line detections they were strongly affected by small number statistics (Simcoe 2006;Ryan-Weber et al 2006;Becker et al 2009). More recently, Ryan-Weber et al (2009) compiled a larger set of IR observations for nine QSOs, with VLT/ISAAC and Keck/NIRSPEC. Their data set contains several sight lines with no C iv detections, which points to a decrease in Ω C iv at z > 5.5.…”

The reionization of carbon

“…It is also relatively straightforward to model because it arises in gas that is optically thin to ionizing photons and lies far enough away from galaxies that the local radiation field may be neglected to first order. Its overall mass density is robustly observed to decline slowly with increasing redshift (Songaila 2001;Becker et al 2009;Ryan-Weber et al 2009;D'Odorico et al 2010D'Odorico et al , 2013Simcoe et al 2011;Becker et al 2011;Cooksey et al 2013). Numerous studies have shown that the observed decline can readily be accommodated by numerical simulations (Oppenheimer & Davé 2006;Oppenheimer et al 2009;Cen & Chisari 2011).…”

“…Intergalactic C IV and C II absorbers are observed along sightlines to quasars out to the highest redshifts probed (Songaila 2001;Ryan-Weber et al 2006;Becker et al 2009;Ryan-Weber et al 2009;Simcoe et al 2011;D'Odorico et al 2013;Becker et al 2011). They occur naturally in cosmological hydrodynamic simulations in which galactic outflows expel metals out to the virial radius and beyond (Theuns et al 2002;Cen et al 2005; Oppenheimer & Davé 2006, 2008Oppenheimer et al 2009;Cen & Chisari 2011), opening up the possibility of using models to interpret them as tracers of star formation, outflows, and the ultraviolet ionizing background (UVB).…”