Extended OVI haloes of starforming galaxies
  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.
