The Metallicity of Intergalactic Gas in Cosmic Voids

• Published in: The Astrophysical journal Vol. 671; no. 1; pp. 146 - 152
• Main Authors: Stocke, John T, Danforth, Charles W, Shull, J. Michael, Penton, Steven V, Giroux, Mark L
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 10-12-2007; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Quasars; galaxies: dwarf; Intergalactic matter; Absorption line; Dwarf galaxies; Metallicity; quasars: absorption lines; intergalactic medium; Intergalactic gas; ultraviolet: general
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/522920

We have used the Hubble STIS and FUSE archives of ultraviolet spectra of bright AGNs to identify intergalactic Ly alpha absorbers in nearby (z less than or equal to 0.1) voids. From a parent sample of 651 Ly alpha absorbers, we identified 61 "void absorbers" located >1.4 h super(-) sub(7) super(1) sub(0) Mpc from the nearest [unk] or brighter galaxy. Searching for metal absorption in high-quality (S/N > 10) spectra at the location of three diagnostic metal lines (O VI lambda 1032, C IV lambda 1548, SI III lambda 1206), 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 Ly alpha absorbers with N sub(H) 6 > 10 super(14) cm super(-2), with 3 sigma equivalent-width limits ranging from 8 mAA (O VI) to 7-15 mAA (C IV) and 4-10 mAA (SI III). Photoionization modeling yields metallicity limits Z < 10 super(-1.8 plus or minus 0.1) Z sub( )[unk] from nondetections of C IV and O VI, some similar to 6 times lower than those seen in Ly alpha /O VI absorbers at z < 0.1. Although the void Ly alpha 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 similar to 3.