Extended OVI haloes of starforming galaxies

Abstract

We consider evolution of metal-enriched gas exposed to a superposition of time-dependent radiation field of a nearby starburst galaxy and nearly invariant (on timescales 100 Myr) extragalactic ionization background. Within nonequilibrium (time-dependent) photoionization models we determine ionization fraction of the OVI ion commonly observed in galactic circumference. We derive then conditions for OVI to appear in absorptions in extended galactic haloes depending on the galactic mass and star formation rate. We have found that the maximum OVI fraction can reach $\sim 0.4-0.9$ under combined action of the galactic and the extragalactic ionizing radiation fields. We conclude that soft X-ray emission with $E\simgt 113$~eV from the stellar population of central starforming galaxies is the main source of such a high fraction of OVI. This circumstance can explain high column densities ${\rm N(OVI)} \sim 10^{14.5 - 15.3}$~cm$^{-2}$ observed in the haloes of starforming galaxies at low redshifts (Tumlinson etal 2011) {\it even} for a relatively low ({ $\sim 0.01-0.1\zsun$}) metallicity. As a result, the requirements to the sources of oxygen in the extended haloes relax to a reasonably conservative level. We show that at $z\simlt 0.5$ ionization kinetics of oxygen in a relatively dense plasma $n\simgt 10^{-4}$ cm$^{-3}$ of outer halo exposed to a low extragalactic ionizing flux is dominated by nonequilibrium effects.