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

Abstract

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 largescale structures, and moreover, the gas is cool (T 0.9 Z� ). Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) data reveal a dramatic cluster of 13 H I Lyman α (Lyα) 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 ρ = 135 h −1 kpc to 1.37 h −1 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 � 10 5 K, and metal lines detected in the Lyα 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 (2σ uncertainty), yet the nearest galaxy which might pollute the intergalactic medium is at least 135 h −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 ◦ 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 = 0.075 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 (ρ = 293 h −1 kpc). The Doppler parameters and H I 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 > 40 km s −1 )L yα lines.