Extended O VI haloes of star-forming galaxies

Abstract

We consider the evolution of metal-enriched gas exposed to a superposition of the time-dependent radiation field of a nearby starburst galaxy and a nearly invariant (on time-scales of 100 Myr) extragalactic ionization background. Using non-equilibrium (timedependent) photoionization models, we determine the ionization fraction of the O VI ion commonly observed in the galactic circumference. We then derive the conditions for O VI to appear in absorption in extended galactic haloes, depending on the galactic mass and star formation rate. We have found that the maximum O VI fraction can reach ~0.4-0.9 under the combined action of galactic and extragalactic ionizing radiation fields. We conclude that soft X-ray emission with E ≳ 113 eV from the stellar population of central star-forming galaxies is the main source of such a high fraction of O VI. This circumstance can explain the high column densities, N(O VI) ~ 1014.5-15.3 cm-2, observed in the haloes of star-forming galaxies at low redshifts even for relatively low (~0.01-0.1 Z⊙) metallicity. As a result, the requirements for sources of oxygen in extended haloes relax to a reasonably conservative level. We show that, at z ≲ 0.5, the ionization kinetics of oxygen in a relatively dense plasma, n ≳ 10-4 cm-3, of an outer halo exposed to a low extragalactic ionizing flux is dominated by non-equilibrium effects.