Discovery of Weak O VI Absorption in Underdense Regions of the Low-redshift Intergalactic Medium

We search for weak O vi absorption in the low-redshift intergalactic medium (IGM) using 82 high signal-to-noise quasar spectra from the Cosmic Origins Spectrograph on the Hubble Space Telescope. From this dataset, we compile a clean sample of 396 intervening Lyman alpha (Ly α ) lines with H i column densities log( N H I cm −2 ) < 14.5 and no individual O vi detections. Stacking at the location of the O vi doublet reveals absorption at >5 σ significance, with equivalent width ( W r 1032 ) = 1.7 ± 0.3 mÅ, corresponding to log( N O VI cm −2 ) = 12.14 ± 0.08. The stacked O vi signal associated with strong Ly α (13.5 ≤ log N H I < 14.5) absorbers is significantly stronger than that associated with weaker Ly α (12.5 ≤ log N H I < 13.5) absorbers. For the subset of 81 broad Ly α absorbers ( b H I > 45 km s −1 ), we obtain a marginal detection of O vi of ∼4 σ . Other than Si iii , detected at 5 σ , no metal lines are found. Cross correlation with galaxies shows that 93% of Ly α absorbers lack an associated bright galaxy (>3 L * ) within 1 Mpc. A more complete low-redshift ( z < 0.25) search shows that 66% of Ly α absorbers lack galaxies down to L * . This suggests that the O vi arises mainly from the diffuse IGM, not the circumgalactic medium. The stacked O vi signal suggests characteristic metallicities of ≈0.01 Z ⊙ under photoionization and ≈0.001 Z ⊙ under collisional ionization, though these estimates are model dependent and assume O vi and H i trace the same phase. This study provides the first observational evidence for metal absorption in low column density Ly α systems that individually show no detectable metals, placing important constraints on metal enrichment of the underdense IGM.