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

Universität Potsdam, Institut für Physik und Astronomie, Karl-Liebknecht-Strasse 24-25, Haus 28, 14476 Potsdam, Germany
e-mail: nadia@astro.physik.uni-potsdam.de

Received: 15 February 2011
Accepted: 14 October 2011

Abstract

To explore the ionization conditions in highly-ionized absorbers at high redshift, we study in detail two intervening O vi absorbers at z ≈ 2 toward 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 typical C iv/O vi Doppler-parameters of b < 10 km s^-1. This implies that the gas temperatures are T < 10⁵ 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 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 that differs 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 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.