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To explore the ionization conditions in highly-ionized absorbers at highredshift we have studied in detail two intervening OVI absorbers at z=2 towardsthe quasar PKS1448-232, based on high (R=75,000) and intermediate (R=45,000)resolution optical VLT/UVES spectra. We find that both absorption systems arecomposed of several narrow subcomponents with CIV/OVI Doppler-parameters b<10km/s, typically. This implies that the gas temperatures are T<10^5 K and thatthe absorbers are photoionized by the UV background. The system at z=2.1098represents a simple, isolated OVI absorber that has only two absorptioncomponents and that is relatively metal-rich (Z\sim 0.6 solar). Ioinizationmodeling implies that the system is photoionized with OVI, CIV, and HIcoexisting in the same gas phase. The second system at z=2.1660 represents acomplicated, multi-component absorption system with eight OVI componentsspanning almost 300 km/s in radial velocity. The photoionization modelingimplies that the metallicity is non-uniform and relatively low (<= 0.1 solar)and that the OVI absorption must arise in a gas phase different from thattraced by CIV, CIII, and HI. Our detailed study of the two OVI systems towardsPKS1448-232 shows that multi-phase, multi-component high-ion absorbers like theone at z=2.1660 require a detailed ionization modeling of the varioussubcomponents to obtain reliable results on the physical conditions andmetal-abundances in the gas.

High-resolution observations of two OVI absorbers at z=2 towards PKS 1448-232