High-metallicity, photoionized gas in intergalactic large-scale filaments

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 < 10(5) K) and has relatively high metallicity (Z > 0.9 Z circle dot). Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) data reveal a dramatic cluster of 13 H I Lyman alpha (Ly alpha) lines within a 1000 km s(-1) interval at z(abs) = 0.0635. We find 10 galaxies at this redshift with impact parameters ranging from rho = 135 h(-1) kpc to 1.37 h(70)(-1) Mpc. The velocities and velocity spread of the Lya 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 Ly alpha lines all indicate a gas temperature T << 105 K, and metal lines detected in the Ly alpha complex also favour photoionized, cool gas. No O VI absorption lines are evident, which is consistent with photoionization models. Remarkably, the metallicity is near-solar, [M/H] = - 0.05 +/- 0.4 (2 sigma uncertainty), yet the nearest galaxy which might pollute the intergalactic medium is at least 135 h(70)(-1) kpc 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 degrees of the sight line suggesting that the QSO line of sight passes near a node in the cosmic web. At z approximate to 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) = 0.075 73 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 (rho = 293 h(70)(-1) kpc). The Doppler parameters and HI column densities of the Ly alpha lines observed along the entire sight line are consistent with those measured towards other low-z QSOs, including a number of broad (b > 40 km s(-1)) Ly alpha lines.