High-resolution observations of two O vi absorbers atz ≈ 2 toward PKS 1448−232

Abstract

To explore the ionization conditions in highly-ionized absorbers at high redshift we have studied in detail two intervening O vi absorbers at z ≈ 2 towards the quasar PKS 1448−232, based on high (R ≈ 75, 000) and intermediate (R ≈ 45, 000) resolution optical VLT/UVES spectra. We find that both absorption systems are composed of several narrow subcomponents with C iv/O vi Doppler-parameters b < 10 km s −1 , typically. This implies that the gas temperatures are T < 10 5 K and that the absorbers are photoionized by the UV background. The system at z = 2.1098 represents a simple, isolated O vi absorber that has only two absorption components and that is relatively metal-rich (Z ∼ 0.6 solar). Ioinization modeling implies that the system is photoionized with O vi, C iv, and H i coexisting in the same gas phase. The second system at z = 2.1660 represents a complicated, multi-component absorption system with eight O vi components spanning almost 300 km s −1 in radial velocity. The photoionization modeling implies that the metallicity is non-uniform and relatively low (≤ 0.1 solar) and that the O vi absorption must arise in a gas phase different from that traced by C iv, C iii, and H i. Our detailed study of the two O vi systems towards PKS 1448−232 shows that multi-phase, multi-component high-ion absorbers like the one at z = 2.1660 require a detailed ionization modeling of the various subcomponents to obtain reliable results on the physical conditions and metal-abundances in the gas.