Broad H I Absorbers as Metallicity-independent Tracers of the Warm-Hot Intergalactic Medium

• Published in: The Astrophysical journal 2010-02, Vol.710 (1), p.613-633
• Main Authors: Danforth, Charles W, Stocke, John T, Shull, J. Michael
• Format: Article
• Language: English
• Subjects: ABSORPTION; APPROXIMATIONS; Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BARYONS; CALCULATION METHODS; COMPUTERIZED SIMULATION; COSMIC RADIO SOURCES; COSMOLOGY; DENSITY; DIMENSIONLESS NUMBERS; Earth, ocean, space; ELECTROMAGNETIC RADIATION; ELEMENTARY PARTICLES; ELEMENTS; Exact sciences and technology; FERMIONS; GALAXIES; HADRONS; HYDROGEN; IONIZATION; LINE WIDTHS; MONTE CARLO METHOD; NONMETALS; PHOTOIONIZATION; PHYSICAL PROPERTIES; QUASARS; RADIATIONS; SIGNAL-TO-NOISE RATIO; SIMULATION; SORPTION; TELESCOPES; ULTRAVIOLET RADIATION; UNIVERSE
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/710/1/613

Thermally broadened Lyalpha absorbers (BLAs) offer an alternative method to highly ionized metal lines for tracing the warm-hot intergalactic medium (WHIM) at T>10{sup 5} K. However, observing BLAs requires data of high quality and accurate continuum definition to detect the low-contrast features, and a good knowledge of the velocity structure to differentiate multiple blended components from a single broad line. Even for well-characterized absorption profiles, disentangling the thermal line width from the various thermal and non-thermal contributors to the observed line width is ambiguous. We compile a catalog of reliable BLA candidates along seven active galactic nucleus sight lines from a larger set of Lyalpha absorbers observed by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST). We compare our measurements based on independent reduction and analysis of the data to those published by other research groups. We examine the detailed structure of each absorber and determine a reliable line width and column density. Purported BLAs are grouped into probable (15), possible (48), and non-BLA (56) categories. Combining the first two categories, we infer a line frequency (dN/dz){sub BLA}=18+-11, comparable to observed O VI absorbers, also thought to trace the WHIM. We discuss the overlap between BLA and O VI absorbers (20%-40%) and the distribution of BLAs in relation to nearby galaxies (O VI detections in BLAs are found closer to galaxies than O VI nondetections). We assume that the line width determined through a multi-line curve of growth (COG) is a close approximation to the thermal line width. Based on 164 measured COG H I line measurements, we statistically correct the observed line widths via a Monte Carlo simulation. Gas temperature and neutral fraction f{sub H{sub I}} are inferred from these statistically corrected line widths and lead to a distribution of total hydrogen columns. Summing the total column density over the total observed path length, we find a BLA contribution to the closure density of OMEGA{sub BLA} = 6.3{sup +1.1}{sub -0.8} x 10{sup -3} h {sup -1}{sub 70} based on 10{sup 4} Monte Carlo simulations of each BLA system. There are a number of critical systematic assumptions implicit in this calculation, and we discuss how each affects our results and those of previously published work. In particular, the most comparable previous study by Lehner et al. gave OMEGA{sub BLA} = 3.6 x 10{sup -3} h {sup -1}{sub 70} or 9