Where Are the Absorbers toward Q2302+029?

We present images and spectroscopy of objects close to the sight line of Q2302+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 system 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 sensitivities 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 ≈6 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 explanation is problematic: in the first case, "associated" absorption at such high ejection velocities is difficult to understand, and challenges the conventional discrimination between intrinsic and extrinsic absorbers; in the second case, the gas must reside in a ~40 h^-1 Mpc long filament aligned along the line of sight in order to reproduce the broad absorption. Since the absorption system is unusual, such a chance alignment might not be unreasonable. Spectroscopy of objects beyond the immediate vicinity of the QSO sight line reveals a galaxy cluster at z = 0.59, which coincides with strong Lyα and more narrow high-ionization lines in the quasar spectrum. Here, too, the lack of galaxies at distances comparable to those found for, e.g., Lyα-absorbing galaxies, suggests that the absorption may arise from intracluster gas unassociated with any individual galaxies.