A large population of metal-rich, compact, intergalactic C IV absorbers - evidence for poor small-scale metal mixing

We carried out a survey for high-metallicity C iv absorbers at redshift z≈ 2.3 in nine high-quality quasar spectra. Using a novel analysis technique, based on detections of C iv lines and automatically determined upper limits on the column densities of H i, C iii, N v and O vi, we find a large population of photoionized, compact (R∼ 102 pc), metal-rich (Z≳ Z⊙) C iv clouds with moderate densities (nH∼ 10−3.5 cm−3), properties that we show are robust with respect to uncertainties in the ionization model. In particular, local sources of ionizing radiation, overabundance of oxygen, departures from ionization equilibrium and collisional ionization would all imply more compact clouds. The clouds are too small to be self-gravitating and pressure confinement is only consistent under special conditions. We argue that the clouds are, in any case, likely to be short-lived and we demonstrate that this implies that the clouds could easily have been responsible for the transport of all metals that end up in the intergalactic medium (IGM). When metal-rich clouds reach pressure equilibrium with the general, photoionized IGM, the heavy elements will still be concentrated in small high-metallicity patches, but they will look like ordinary, low-metallicity absorbers. We conclude that intergalactic metals are poorly mixed on small scales and that nearly all of the IGM, and thus the universe, may therefore be of primordial composition.