The O VI absorbers toward PG 0953+415: High-metallicity, cosmic-web gas far from luminous galaxies

The spectrum of the low-redshift QSO PG 0953+415 (Z_QSO = 0.234) shows two strong, intervening O VI absorption systems. To study the nature of these absorbers, we have used the Gemini Multi-Object Spectrograph to conduct a deep spectroscopic galaxy redshift survey in the 5′ × 5′ field centered on the QSO. This survey is fully complete for r′ < 19.7 and is 73% complete for r′ < 21.0. We find three galaxies at the redshift of the higher z O VI system (z_abs = 0.14232), including a galaxy at projected distance p = 155 h-₇₀^-1 kpc. We find no galaxies in the Gemini field at the redshift of the lower z O VI absorber (z _abs = 0.06807), which indicates that the nearest galaxy is more than 195 h₇₀^-1 kpc away or has L < 0.04L*. Previous shallower surveys covering a larger field have shown that the z_abs = 0.06807 O VI absorber is affiliated with a group or filament of galaxies, but the nearest known galaxy has p = 736 h₇₀^-1 kpc. The z _abs = 0.06807 absorber is notable for several reasons. The absorption profiles reveal simple kinematics indicative of quiescent material. The H i line widths and good alignment of the H I and metal lines favor photoionization, and moreover, the column density ratios imply a high metallicity: [M/H] = -0.3 ±0.12. The z_abs = 0.14232 O VI system is more complex and less constrained but also indicates a relatively high metallicity. Using galaxy redshifts from the Sloan Digital Sky Survey, we show that both of the PG 0953+415 O VI absorbers are located in large-scale filaments of the cosmic web. Evidently, some regions of the web filaments are highly metal enriched. We discuss the origin of the high-metallicity gas and suggest that the enrichment might have occurred long ago (at high z).