Probing the Warm Hot IGM with Broad Lyα Absorbers

Type Conference Paper
Names Charles Danforth, J. T. Stocke, J. Shull, B. D. Savage, K. R. Sembach
Proceedings Title Bulletin of the American Astronomical Society
Conference Name American Astronomical Society, AAS Meeting #215, #464.23
Volume 42
Pages 502
Date January 1, 2010
Library Catalog NASA ADS
Abstract Broad Lyman alpha absorbers (BLAs) offer an alternative method to highly-ionized metal lines for tracing the warm-hot intergalactic medium (WHIM) at T>105 K. Such features are thermally broadened to line widths of greater than 40 km s-1 and are typically weak (N<1014 cm-2). Observing such low-contrast spectral features and clearly identifying lines likely shaped by thermal broadening requires high-quality data and accurate continuum placement. Thanks to the high sensitivity of the Cosmic Origins Spectrograph (COS), high signal-to-noise (S/N) far-UV spectra of faint AGN are now attainable in modest observing times. We present early COS G130M/G160M observations (lambda=1130-1800 A) of the quasars PKS 0405-123 (z=0.57, S/N=30-50) and 1ES 1553+113 (z 0.40, S/N=15-20). Both sight lines are rich in IGM absorption systems and the high S/N allows us to catalog several dozen BLA features. We compare the new COS observations of PKS 0405-123 with archival STIS/E140M observations (S/N=10-15) and find qualitative agreement with roughly half of the published BLA systems. As comparable previous observations of 1ES 1553+113 do not exist, we present several new BLA candidates along this sight line. We measure the BLA detection frequency, determine the coincidence of BLAs with other WHIM tracers such as OVI, and estimate the contribution of BLAs to the total baryon density of the universe. Finally, we outline some of the biases and systematic assumptions inherent in BLA WHIM surveys and discuss how each affects the search for missing baryons.
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