Publication Date:
2014-12-03
Description:
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 (time-dependent) 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, ${N(\rm O\,\small {vi})} \sim 10^{14.5\mbox{--}15.3}$ 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.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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