Multiphase plasma in sub-damped Lyα systems:: A hidden metal reservoir

We present a VLT UVES spectrum of a proximate sub-damped Ly alpha (sub-DLA) system at z(abs =) 2.65618 toward the quasar Q0331-4505 (z(qso) = 2.6785 +/- 0.0030). Absorption lines of O I, Si II, Si III, Si IV, C II, C III, qso C IV, Fe II, Al II, and O VI are seen in the sub-DLA, which has a neutral hydrogen column density log N-H I = 19.82 +/- 0.05. The absorber is at a velocity of 1820 +/- 250 km s(-1) from the quasar; however, its low metallicity [O/H] = -1.64 +/- 0.07, lack of partial coverage, lack of temporal variations between observations taken in 2003 and 2006, and nondetection of N v imply the absorber is not a genuine intrinsic system. By measuring the O VI column density and assuming equal metallicities in the neutral and ionized gas, we determine the column density of hot ionized hydrogen in this sub-DLA, and in two other sub-DLAs with O vi drawn from the literature. Coupling this with determinations of the typical amount of warm ionized hydrogen in sub-DLAs, we confirm that sub-DLAs are a more important metal reservoir than DLAs, in total comprising at least 6% - 22% of the metal budget at. z approximate to 2.5.