Highly ionized absorbers at high redshift

Abstract

We investigate the existence of a metal-rich OVI population in 10 high spectral resolution quasar spectra and define observational criteria for this class of absorbers. The low temperatures of nearly half of all OVI absorbers (redshifts 2.0 to 2.6), implied by their line widths, are too low for collisional ionization to be a dominant process. We estimate the oxygen abundance under the assumption of photoionization; a striking result is the bimodal distribution of [O/H] with median values close to 0.01 (0.5) solar for the metal-poor (metal-rich) populations. We present the first estimate of the OVI column density distribution and fit a single power-law index of ~-1.7 and a normalization of f(N)= 2.3e(-13) at log N(OVI) ~ 13.5, both with a ~30% uncertainty. The value of the index is similar to that found for CIV surveys, whereas the normalization factor is about ten times higher. We use f(N) to derive dn/dz and the cosmic density, Omega_b(OVI), selecting a limited column density range not strongly affected by incompleteness or sample variance. We find some decline of dn/dz with z. The cosmic OVI density derived from f(N), Omega_b(OVI) ~ 3.5e(-7), is 2.3 times higher than the value estimated using the observed OVI sample (of which the metal-rich population contributes ~35%), easing the problem of missing metals at high z (~1/4 of the produced metals) but not solving it. We find that the majority of the metal-rich absorbers are located within ~450 km/s of strong Ly-a lines and show that, contrary to the metal-poor absorbers, this population cannot be in hydrostatic equilibrium. All of the OVI absorber properties imply that there are two distinct populations: metal-poor absorbers tracing the intergalactic medium and metal-rich absorbers associated with active sites of star formation.