2006MNRAS.367..139A

We present high-resolution ultraviolet spectra of absorption-line systems towards the low-z quasi-stellar object (QSO) HS 0624+6907 (z_QSO= 0.3700). Coupled with ground-based imaging and spectroscopic galaxy redshifts, we find evidence that many of these absorbers do not arise in galaxy haloes but rather are truly intergalactic gas clouds distributed within large-scale structures, and moreover, the gas is cool (T < 10⁵ K) and has relatively high metallicity (Z > 0.9Z_☉). Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) data reveal a dramatic cluster of 13 Hi Lyman α (Lyα) lines within a 1000 km/s interval at z_abs= 0.0635. We find 10 galaxies at this redshift with impact parameters ranging from ρ= 135h^–1₇₀kpc to 1.37h^–1₇₀ Mpc. The velocities and velocity spread of the Lyα lines in this complex are unlikely to arise in the individual haloes of the nearby galaxies; instead, we attribute the absorption to intragroup medium gas, possibly from a large-scale filament viewed along its long axis. Contrary to theoretical expectations, this gas is not the shock-heated warm-hot intergalactic medium (WHIM); the width of the Lyα lines all indicate a gas temperature T≪ 10⁵ K, and metal lines detected in the Lyα complex also favour photoionized, cool gas. No Ovi absorption lines are evident, which is consistent with photoionization models. Remarkably, the metallicity is near-solar, [M/H]=-0.05±0.4 (2σ uncertainty), yet the nearest galaxy which might pollute the intergalactic medium is at least 135h^–1₇₀kpc away. Tidal stripping from nearby galaxies appears to be the most likely origin of this highly enriched, cool gas. More than six Abell galaxy clusters are found within 4° of the sight line suggesting that the QSO line of sight passes near a node in the cosmic web. At z~ 0.077, we find absorption systems as well as galaxies at the redshift of the nearby clusters Abell 564 and Abell 559. We conclude that the sight line pierces a filament of gas and galaxies feeding into these clusters. The absorber at z_abs= 0.07573 associated with Abell 564/559 also has a high metallicity with [C/H] > -0.6, but again the closest galaxy is relatively far from the sight line (ρ= 293h^–1₇₀kpc). The Doppler parameters and Hi column densities of the Lyα lines observed along the entire sight line are consistent with those measured towards other low-z QSOs, including a number of broad (b > 40km/s) Lyα lines.