The Nature and Origin of Low-Redshift O VI Absorbers

TL;DR

This paper investigates the origin and nature of low-redshift O VI absorbers using cosmological simulations, revealing they are mainly photo-ionized, associated with enriched regions, and influenced by turbulence and recent outflows.

Contribution

It introduces a comprehensive simulation-based analysis of O VI absorbers, highlighting the roles of turbulence, enrichment history, and environment in their properties.

Findings

O VI is predominantly photo-ionized at T≈10^4.2 K.

Turbulence is necessary to match observed O VI properties.

Strong absorbers are often collisionally ionized and linked to recent outflows.

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. We consider the effects of ionization background variations, non-equilibrium ionization and cooling, uniform metallicity, and small-scale (sub-resolution) turbulence. Our main results are 1) IGM O VI is predominantly photo-ionized with T= 10^(4.2+/-0.2) K. A key reason for this is that O VI absorbers preferentially trace over-enriched regions of the IGM at a given density, which enhances metal-line cooling such that absorbers can cool within a Hubble time. As such, O VI is not a good tracer of the WHIM. 2) The predicted O VI properties fit observables only if sub-resolution turbulence is added. The required turbulence increases with O VI absorber strength such that stronger absorbers arise from more recent outflows with turbulence dissipating on the order of a Hubble time. The amount of turbulence is consistent with other examples of turbulence observed in the IGM and galactic halos. 3) Metals traced by O VI and H I do not trace exactly the same baryons, but reside in the same large-scale structure. Observed alignment statistics are reproduced in our simulations. 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 are more likely to be collisionally ionized, tracing more recent enrichment (<2 Gyr) within or near galactic halos.