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 obtained with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. From this dataset, we compile a clean sample of 396 intervening Lyman-alpha (Lya) absorption lines with H I column densities log (N_HI) < 14.5, all of which lack individual O VI absorption with log (N_OVI ) > 13. We perform a spectral stacking analysis at the expected location of the O VI doublet, revealing O VI absorption with a statistical significance greater than 5$\sigma$, and measure an equivalent width of 1.7 $\pm$ 0.3 mA, corresponding to log (N_OVI ) = 12.14 $\pm$ 0.08. The stacked O VI absorption signal associated with strong Lya absorbers (13.5 <= log N_HI < 14.5) is significantly stronger than that associated with weaker Lya absorbers (12.5 <= log N_HI < 13.5). For the subset of 81 broad Lya absorbers (BLAs; b(HI) > 45 km/s), we obtain a marginal $\sim$3 $\sigma$ O VI detection. Other than Si III, detected at 5$\sigma$, no associated metal lines are found. Cross-correlation of the Lya absorbers with galaxies indicates that 93% of these absorbers are not associated with bright galaxies within 1 Mpc, implying that the detected O VI originates in the diffuse IGM rather than the circumgalactic medium. The stacked O VI signal suggests characteristic metallicities of $\sim 0.01\,Z_{\odot}$ under photoionisation and $\sim 0.001\,Z_{\odot}$ under collisional ionisation conditions, though these estimates are model-dependent and assume that O VI and H I trace the same phase. This study provides the first observational evidence for metal absorption in low-column-density Lya systems that individually exhibit no detectable metals, placing important constraints on the metal enrichment of the underdense IGM.