Where are the absorbers toward Q2302+029?

We present images and spectroscopy of objects close to the sight line of Q2 30+029 in order to search for galaxies responsible for the remarkable z = 0 .7 high-ionization absorption-line system found by Jannuzi et al. This syst em shows "normal" narrow O VI, N V, and C IV lines superimposed on broader (3000-5000 km s(-1) wide), unsaturated absorption troughs some 56,000 km s( -1) away from the QSO emission redshift (z(em) = 1.052). Despite reaching s ensitivities sufficient to detect 1/10L* galaxies in the optical and 1/20L* in the infrared, we are unable to detect any obvious bright galaxies that might be responsible for the absorption beyond approximate to6 h(-1) kpc of the sight line. This supports the hypotheses that the absorption is either intrinsic to the QSO or arises in intracluster gas. Adopting either explan ation is problematic: in the first case, "associated" absorption at such hi gh ejection velocities is difficult to understand, and challenges the conve ntional discrimination between intrinsic and extrinsic absorbers; in the se cond case, the gas must reside in a similar to 40 h(-1) Mpc long filament a ligned along the line of sight in order to reproduce the broad absorption. Since the absorption system is unusual, such a chance alignment might not b e unreasonable. Spectroscopy of objects beyond the immediate vicinity of th e QSO sight line reveals a galaxy cluster at z = 0.59, which coincides with strong Ly alpha and more narrow high-ionization lines in the quasar spectr um. Here, too, the lack of galaxies at distances comparable to those found for, e.g., Ly alpha -absorbing galaxies, suggests that the absorption may a rise from intracluster gas unassociated with any individual galaxies.