2012A&A...538A..85D

To explore the ionization conditions in highly-ionized absorbers at high redshift, we study in detail two intervening OVI absorbers at z∼2 toward the quasar PKS1448-232, based on high (R∼75000) and intermediate (R∼45000) resolution optical VLT/UVES spectra. We find that both absorption systems are composed of several narrow subcomponents with typical CIV/OVI Doppler-parameters of b<10km/s. This implies that the gas temperatures are T<10⁵K and that the absorbers are photoionized by the UV background. The system at z=2.1098 represents a simple, isolated OVI absorber that has only two absorption components and is relatively metal-rich (Z∼0.6 solar). Ioinization modeling implies that the system is photoionized with OVI, CIV, and HI coexisting in the same gas phase. The second system at z=2.1660 represents a complicated, multi-component absorption system with eight OVI components spanning almost 300km/s in radial velocity. The photoionization modeling implies that the metallicity is non-uniform and relatively low (≤0.1solar) and that the OVI absorption must arise in a gas phase that differs from that traced by CIV, CIII, and HI. Our detailed study of the two OVI systems towards PKS1448-232 shows that multi-phase, multi-component high-ion absorbers similar to the one at z=2.1660 can be described by applying a detailed ionization modeling of the various subcomponents to obtain reliable measurements of the physical conditions and the metal abundances in the gas.