The nature and origin of low-redshift O vi absorbers

Abstract

The O VI ion observed in quasar absorption-line spectra is the most accessible tracer of the cosmic metal distribution in the low-redshift ( z< 0.5) intergalactic medium (IGM). We explore the nature and origin of O VI absorbers using cosmological hydrodynamic simulations including galactic outflows with a range of strengths. We consider the effects of ionization background variations, non-equilibrium ionization and cooling, uniform metallicity and small-scale (subresolution) turbulence. Our main results are as follows. (1) IGM O VI is predominantly photoionized with T ≈ 10 4.2±0.2 K. A key reason for this is that O VI absorbers preferentially trace overenriched (by ∼× 5) regions of the IGM at a given density, which enhances metal-line cooling such that absorbers can cool to photo-ionized temperatures within a Hubble time. As such, O VI is not a good tracer of the warm-hot intergalactic medium. (2) The predicted O VI properties fit observables if and only if sub-resolution turbulence is added, regardless of any other model variations. The required turbulence increases with O VI absorber strength. Stronger absorbers arise from more recent outflows, so qualitatively this can be understood if IGM turbulence dissipates on the order of a Hubble time. The amount of turbulence is consistent with other examples of turbulence observed in the IGM and galactic haloes. (3) Metals traced by O VI and H I do not trace exactly the same baryons, but reside in the same large-scale structure. Our simulations reproduce observed alignment statistics between O VI and H I, yet aligned absorbers typically have O VI arising from cooler gas, and for stronger absorbers lower densities, than H I. Owing to peculiar velocities dominating the line structure, coincident absorption often arises from spatially distinct gas. (4) Photo-ionized O VI traces gas in a variety of environments, and is not directly associated with the nearest galaxy, though is typically nearest to ∼0.1L∗ galaxies. Weaker O VI components trace some of the oldest cosmic metals. (5) Very strong absorbers (EW 100 mA) are more likely to be collisionally ionized, tracing more recent enrichment (2 Gyr) within or near galactic haloes.