2007ApJ...671..146S

We have used the Hubble STIS and FUSE archives of ultraviolet spectra of bright AGNs to identify intergalactic Lyα absorbers in nearby (z≤0.1) voids. From a parent sample of 651 Lyα absorbers, we identified 61 ``void absorbers'' located >1.4 h^–1₇₀ Mpc from the nearest L^* or brighter galaxy. Searching for metal absorption in high-quality (S/N>10) spectra at the location of three diagnostic metal lines (O VI λ1032, C IV λ1548, Si III λ1206), we detected no metal lines in any individual absorber, or in any group of absorbers using pixel co-addition techniques. The best limits on metal-line absorption in voids were set using four strong Lyα absorbers with N_HI_>10¹⁴/cm2, with 3 σ equivalent-width limits ranging from 8 mÅ (O VI) to 7-15 mÅ (C IV) and 4-10 mÅ (Si III). Photoionization modeling yields metallicity limits Z<10^–1.8±0.4 Z_☉ from nondetections of C IV and O VI, some ∼6 times lower than those seen in Lyα/O VI absorbers at z<0.1. Although the void Lyα absorbers could be pristine material, considerably deeper spectra are required to rule out a universal metallicity floor produced by bursts of early star formation, with no subsequent star formation in the voids. The most consistent conclusion derived from these low-z results and similar searches at z=3-5 is that galaxy filaments have increased their mean IGM metallicity by factors of 30-100 since z∼3.