The O VI Absorbers toward PG 0953+415: High-Metallicity, Cosmic-Web Gas Far from Luminous Galaxies

Abstract

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 ρ=155 h^-1₇₀ 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 h^-1₇₀ 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 ρ=736 h^-1₇₀ 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).

Based on observations with (1) the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555 (2) the NASA-CNES-ESA Far Ultraviolet Spectroscopic Explorer (FUSE), operated for NASA by Johns Hopkins University under NASA contract NAS 5-32985 and (3) the Gemini Observatory, which is operated by AURA, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (US), the Particle Physics and Astronomy Research Council (UK), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).