The Hubble Space Telescope quasar absorption line key project: The unusual absorption-line system in the spectrum of PG 2302+029 - Ejected or intervening?

We report the discovery of a high-ionization broad absorption line system at a redshift of z_abs = 0.695 in the spectrum of the z_em = 1.052 radio-quiet quasar PG 2302+029. Broad absorption with FWHM from 3000 to 5000 km s^-1 is detected from C IV, N V, and O VI in Hubble Space Telescope (HST) Faint Object Spectrograph spectra of the quasar. A narrow-line system (FWHM ∼ 250 km s^-1) at z_abs = 0.7016 is resolved from the broad blend and includes absorption by Lyα and the C IV, N V, and O VI doublets. No absorption by low-ionization metal species (e.g., Si II and Mg II) is detected in the HST or ground-based spectra for either the broad or the narrow system. The centroids of the broad system lines are displaced by ∼56,000 km s^-1 to the blue of the quasar's broad emission lines. The reddest extent of the broad-line absorption is more than 50,000 km s^-1 from the quasar. The properties of this system are unprecedented, whether it is an intervening or an ejected system.